Mass Transport Limitations Research Articles

Single‐atom Pt on carbon nanotubes for selective electrocatalysis

AbstractUtilizing supported single atoms as catalysts presents an opportunity to reduce the usage of critical raw materials such as platinum, which are essential for electrochemical reactions such as hydrogen oxidation reaction (HOR). Herein, we describe the synthesis of a Pt single electrocatalyst inside single‐walled carbon nanotubes (SWCNTs) via a redox reaction. Characterizations via electron microscopy, X‐ray photoelectron microscopy, and X‐ray absorption spectroscopy show the single‐atom nature of the Pt. The electrochemical behavior of the sample to hydrogen and oxygen was investigated using the advanced floating electrode technique, which minimizes mass transport limitations and gives a thorough insight into the activity of the electrocatalyst. The single‐atom samples showed higher HOR activity than state‐of‐the‐art 30% Pt/C while almost no oxygen reduction reaction activity in the proton exchange membrane fuel cell operating range. The selective activity toward HOR arose as the main fingerprint of the catalyst confinement in the SWCNTs.

(Invited) Dendrite Formation in Lithium Electrodeposition – Insights from Experiments and Modeling

High-energy density batteries are essential for powering future electric vehicles and electric aircrafts. The Li-metal anode offers the highest theoretical specific energy among practically available anode materials; however, a major hurdle in the progress towards rechargeable Li-metal batteries is the dendritic electrodeposition of Li metal during the battery charging process. The physicochemical mechanisms behind Li dendrite formation have been a topic of rich and fervent research, but many questions remain presently unanswered. Among these, a key question of significance to battery charging is: at what time instant is Li dendritic growth initiated? In the present talk, this question will be addressed through a combination of experiments and modeling. Chronopotentiometry during galvanostatic Li electrodeposition coupled with in situ optical microscopy reveals that the first emergence of Li dendrites corresponds to the time instant when the surface overpotential reaches a maximum.1 Electrochemical impedance spectroscopy shows that the increase in the surface overpotential preceding this maximum is due to gradual growth of the solid electrolyte interphase (SEI). This SEI layer growth induces Li+ concentration depletion at the electrode-SEI interface, promoting amplification of surface asperities and thus dendrites that rupture the SEI. A transport model incorporating Li+ diffusion across a growing SEI layer is developed, and shown to predict Li dendrite onset times that are in good agreement with experimental observations over a wide range of current densities, temperature, and initial SEI thicknesses.2 A key conclusion of our work is that liquid-phase mass transport limitations, often characterized by the Sand’s equation, do not reliably predict the onset of Li dendrites.3 Rather, transport characteristics and associated instabilities within solid-phase surface films on Li are predominantly responsible for dendrite formation. Our work provides clues for how dendrite formation could be arrested for enabling rechargeable Li-metal anodes for use in next-generation batteries.

Perspective on the mechanism of mass transport-induced (tip-growing) Li dendrite formation by comparing conventional liquid organic solvent with solid polymer-based electrolytes

A major challenge of Li metal electrodes is the growth of high surface area lithium during Li deposition with a variety of possible shapes and growing mechanisms. They are reactive and lead to active lithium losses, electrolyte depletion and safety concerns due to a potential risk of short-circuits and thermal runaway. This work focuses on the mechanism of tip-growing Li dendrite as a particular high surface area lithium morphology. Its formation mechanism is well-known and is triggered during concentration polarization, i.e. during mass (Li+) transport limitations, which has been thoroughly investigated in literature with liquid electrolytes. This work aims to give a stimulating perspective on this formation mechanism by considering solid polymer electrolytes. The in-here shown absence of the characteristic “voltage noise” immediately after complete concentration polarization, being an indicator for tip-growing dendritic growth, rules out the occurrence of the particular tip-growing morphology for solid polymer electrolytes under the specific electrochemical conditions. The generally poorer kinetics of solid polymer electrolytes compared to liquid electrolytes imply lower limiting currents, i.e. lower currents to realize complete concen­tration polarization. Hence, this longer-lasting Li-deposition times in solid polymer electro­lytes are assumed to prevent tip-growing mechanism via timely enabling solid electrolyte interphase formation on fresh Li deposits, while, as stated in previous literature, in liquid electrolytes, Li dendrite tip-growth process is faster than solid electrolyte interphase forma­tion kinetics. It can be reasonably concluded that tip-growing Li dendrites are in general practically unlikely for both, (i) the lower conducting electrolytes like solid polymer electro­lytes due to enabling solid electrolyte interphase formation and (ii) good-conducting electro­lytes like liquids due to an impractically high current required for concentration polarization.

Insight into the Physicochemical Properties of Co-Based Catalysts in Fischer–Tropsch Synthesis

The effect of the different supports and catalyst-reducing agents on the Fischer–Tropsch (FT) reaction was investigated. The large surface area SiO2 support with a smaller pore volume deposited fine, evenly distributed Co3O4. Cubic-shaped Co3O4 appeared in clusters on the TiO2 support, whereas Co3O4 existed as single large particles on the Al2O3 support. The activity data obtained were discussed in terms of cluster size, particle size, particle shape, and mass transport limitations. The SiO2-supported catalysts showed a higher activity for the formation of paraffinic products when reduced in H2 at 250 °C. This is attributed to the formation of the CoO-Co active bond, which enhanced the activation of CO and the hydrogenation reactions. A higher activity was observed for the TiO2-supported catalyst at a higher reduction temperature (350 °C) when the mass of Co metal was higher. It afforded more paraffinic products due to enhanced secondary hydrogenation of olefins at higher reaction rates. The large Co3O4 supported on Al2O3 showed the least activity at both reduction temperatures due to strong metal-support interactions. The H2-reduced catalysts exhibited superior activity compared to all the syngas-reduced catalysts. Syngas reduction led to surface carbon deposition and the formation of surface carbides which suppressed the hydrogenation reactions and are selective to olefinic products.

Ultrafast deposition of faceted lithium polyhedra by outpacing SEI formation.

Electrodeposition of lithium (Li) metal is critical for high-energy batteries1. However, the simultaneous formation of a surface corrosion film termed the solid electrolyte interphase (SEI)2 complicates the deposition process, which underpins our poor understanding of Li metal electrodeposition. Here we decouple these two intertwined processes by outpacing SEI formation at ultrafast deposition current densities3 while also avoiding mass transport limitations. Byusing cryogenic electron microscopy4-7, we discover the intrinsic deposition morphology of metallic Li to be that of a rhombic dodecahedron, which is surprisingly independent of electrolyte chemistry or current collector substrate. In a coin cell architecture, these rhombic dodecahedra exhibit near point-contact connectivity with the current collector, which can accelerate inactive Li formation8. We propose a pulse-current protocol that overcomes this failure mode by leveraging Li rhombic dodecahedra as nucleation seeds, enabling the subsequent growth of dense Li that improves battery performance compared with a baseline. While Li deposition and SEI formation have always been tightly linked in past studies, our experimental approach enables new opportunities to fundamentally understand these processes decoupled from each other and bring about new insights to engineer better batteries.

Accumulation of Antifreeze Proteins on Ice Is Determined by Adsorption.

Antifreeze proteins (AFPs) facilitate the survival of diverse organisms in frigid environments by adsorbing to ice crystals and suppressing their growth. The rate of AFP accumulation on ice is determined by an interplay between AFP diffusion from the bulk solution to the ice-water interface and the subsequent adsorption of AFPs to the interface. To interrogate the relative importance of these two processes, here, we combine nonequilibrium fluorescence experiments with a reaction-diffusion model. We find that as diverse AFPs accumulate on ice, their concentration in the aqueous solution does not develop a gradient but remains equal to its bulk concentration throughout our experiments. These findings lead us to conclude that AFP accumulation on ice crystals, which are smaller than 100 μm in radius, is not limited by the diffusion of AFPs, but by the kinetics of AFP adsorption. Our results imply that mass transport limitations do not hinder AFPs from performing their biological function.

Operando investigations of proton exchange membrane fuel cells performance during air interruptions in dry and humidified conditions

Activating proton exchange membrane fuel cells is crucial to systematically build proton conductive pathways, remove contaminants from the electrode surface and obtain the highest performances, with air interruption emerging as one of the most powerful methods. While this approach enables fast activation and considerable recovery after extended periods of shutdown or operation, the impact of relative humidity on this process has not been explored. Herein, the impact of the relative humidity on the air interruption is investigated using an array of operando methods such as polarization, electrochemical impedance spectroscopy, and high-resolution localized current and mapping. It is found that while air interruptions in humidified conditions can improve performance, they rapidly reduce them in dry conditions. Specifically, air interruptions in fully humidified conditions improve ions, charges, and gas diffusion to the active sites, which may be attributed to the removal of platinum oxides increasing active site availability. However, air interruptions under low relative humidity rapidly reduce the cell voltage by increasing charge transfer and mass transport limitations, while causing extensive overheating as the cell is starved of oxygen and hydrogen might be generated. Overall, this work reveals the potential, yet limitations of air interruptions in operando hydrogen fuel cells.

Modular assembly of microswimmers with liquid compartments.

Artificial microswimmers, i.e. colloidal scale objects capable of self-propulsion, have garnered significant attention due to their central role as models for out of equilibrium systems. Moreover, their potential applications in diverse fields such as biomedicine, environmental remediation, and materials science have long been hypothesized, often in conjunction with their ability to deliver cargoes to overcome mass transport limitations. A very efficient way to load molecular cargoes is to disperse them in a liquid compartment, however, fabricating microswimmers with multiple liquid compartments remains a significant challenge. To address this challenge, we present a modular fabrication platform that combines microfluidic synthesis and sequential capillarity-assisted particle assembly (sCAPA) for microswimmers with various liquid compartments. We demonstrate the synthesis of monodisperse, small polymer-based microcapsules (Ø = 3-6μm) with different liquid cargoes using a flow-focusing microfluidic device. By employing the sCAPA technique, we assemble multiple microcapsules into microswimmers with high precision, resulting in versatile microswimmers with multiple liquid compartments and programmable functionalities. Our work provides a flexible approach for the fabrication of modular microswimmers, which could potentially actively transport cargoes and release them on demand in the future.

A simple electrodeposition problem and the abundant knowledge it subserves to teach

This paper presents a handful of electrochemical experiments related to one single system that opens up the way to teach a bunch of topics related to physical and inorganic chemistry, hence serving experience-oriented education of otherwise hard-to-understand fields. The key idea is the electrodeposition of metals from the same bath containing Cu2+ and Ni2+ ions onto electrodes that differ from each other either in size or position within the electrochemical cell. With the proper optimization of the current densities, pure Cu and a Ni-rich coating with shiny silver colour can be obtained on electrodes of large and small surface area, respectively. It will be explained in detail how to drive the discussion on the experiment so that the audience learns which processes can be treated as parallel ones (i.e. the deposition of two metals, Cu and Ni), the mass transport taking place in serial with the electrochemical reaction, and the occurrence of the mass transport limitation in an electrode process. Didactic aspects of the experiment are presented for both high school and undergraduate levels, and control experiments are also suggested to verify the conclusions achieved. Collateral topics of chemistry and materials science that can be brought up in connection with the experiment are also enumerated.

Accelerating cryoprotectant delivery using vacuum infiltration

The ability to cryopreserve bone marrow within the vertebral body (VB) would offer significant clinical and research benefits. However, cryopreservation of large structures, such as VBs, is challenging due to mass transport limitations that prevent the effective delivery of cryoprotectants into the tissue. To overcome this challenge, we examined the potential of vacuum infiltration, along with carbonation, to increase the penetration of cryoprotectants. In particular, we hypothesized that initial exposure to high-pressure carbon dioxide gas would introduce bubbles into the tissue and that subsequent vacuum cycling would cause expansion and contraction of the bubbles, thus enhancing the transport of cryoprotectant into the tissue. Experiments were carried out using colored dye and agarose gel as a model revealing that carbonation and vacuum cycling result in a 14% increase in dye penetration compared to the atmospheric controls. Experiments were also carried out by exposing VBs isolated from human vertebrae to 40% (v/v) DMSO solution. CT imaging showed the presence of gas bubbles within the tissue pores for carbonated VBs as well as control VBs. Vacuum cycling reduced the bubble volume by more than 50%, most likely resulting in replacement of this volume with DMSO solution. However, we were unable to detect a statistically significant increase in DMSO concentration within the VBs using CT imaging. This research suggests that there may be a modest benefit to carbonation and vacuum cycling for introduction of cryoprotectants into larger structures, like VBs.

The Burmese Jade Mines belt: origins of jadeitites, serpentinites, and ophiolitic peridotites and gabbros

Ophiolitic peridotites in Myanmar (Burma) occur along three major tectonic zones: the Kaleymyo–Nagaland suture along the Indo-Burman Ranges, the Jade Mines belt and the Tagaung–Mytkyina belt. These belts all show harzburgite–lherzolite–dunite peridotites, but the Hpakan-Taw Maw region (Jade Mines belt) also hosts jadeitites, including pure jadeite, mawsitsit (Cr-rich jadeite) kosmochlore (Cr-rich clinopyroxene) and albitite. Jadeitites with high Na and Al contents require either very unusual Al-rich, Si-poor protoliths or extensive fluid metasomatism, or both. The Hpakan jadeitites formed by Na-, Al- (and Si-) metasomatic alteration of pyroxenite–wehrlite intrusions into harzburgite–dunite from widespread fluid alteration. The fluids could have been derived from a mid-Jurassic intermediate pressure subduction event during ophiolite formation and emplacement. In the Lake Indawgyi area, normal ophiolitic peridotites, including harzburgite and dunite with pyroxenite veins, have not been jadeitized. Gabbros related to the Jade Mines ophiolite gave a U–Pb zircon age of 169.71 ± 1.3 Ma (MSWD 2.2), a similar timing to the Myitkyina ophiolite (173 Ma) to the east, suggesting that the ophiolite belts were originally continuous. The jade ‘boulders’ in the Uru conglomerate beds at Hpakan have also resulted from normal in situ serpentinization weathering processes, followed by limited fluvial mass transport processes along the Uru River. Supplementary material: U–Pb zircon data are available at https://doi.org/10.6084/m9.figshare.c.6655269

Kraft cooking of birch wood chips: differences between the dissolved organic material in pore and bulk liquor

Abstract The delignification of birch chips during kraft pulping was investigated, targeting both the impregnation and cooking steps. Wood chips were impregnated using white liquor, white liquor + NaCl, water or NaCl aqueous solution. Then, the chips were cooked in batch autoclaves applying the same constant composition cooking conditions for all samples. Pulp and two fractions of black liquor (bulk liquor and centrifuged liquor representing the liquor inside the wood chips and fibers) were collected after different pulping times and analyzed for lignin and carbohydrate content. The dissolved wood components were precipitated from selected samples and characterized with respect to composition, molecular weight distribution and structural motifs. Cooking chemicals in the impregnation liquors led to faster delignification and xylan removal during cooking. Higher contents of lignin and xylan were measured in the lumen than in the bulk. The concentration profiles also showed accumulation of dissolved material in the lumen over time, suggesting significant mass transport limitation from lumen to bulk. Further analysis revealed higher fragmentation/degradation of dissolved material with increasing pulping time and in the bulk when compared to the lumen liquor, as demonstrated by the lower molecular weights and the changes in chemical shifts in the NMR spectra.

First-Principles Investigation into the Contributions of ORR and HER in Magnesium Corrosion

Recent experiments have demonstrated that oxygen reduction reaction (ORR) can contribute significantly to Mg corrosion in addition to hydrogen reduction reaction (HER). However, its contributions at various stages of Mg corrosion remains unclear. Using DFT calculations, we investigate ORR and HER on both pristine and Fe/Mn alloyed Mg, MgO and Mg(OH)2 during Mg corrosion. We found that both HER and ORR on pristine and alloyed Mg contribute to the overall cathodic current density: HER contributes much more significantly than ORR, while the ORR contribution is up to the mass transport limit and would diminish over time. In addition, we found that ORR activities on Fe/Mn alloyed MgO are high, suggesting an alternate source of cathodic ORR contribution during the whole course of Mg corrosion. Excellent agreements are found between our findings and a recent experiment.

Tuning High-Density Polyethylene Hydrocracking through Mordenite Zeolite Crystal Engineering

We investigate the hydrocracking of high-density polyethylene using a bifunctional Pt/Al2O3 and modified mordenite acid catalyst. Mass transport limitations impact polymer diffusion into the mordenite pore complex. Initial reaction intermediates are formed on the zeolite’s outer surface. Intercrystallite open-end mesopores improve the diffusion of reaction intermediates deeper into the crystal. Recrystallization and desilication of mordenite lead to a higher polymer conversion and shift the product distribution maximum from pentanes to hexanes and heptanes. The nature of mesopores (occluded or open) and total Brønsted acidity significantly impact zeolite activity and selectivity.

Spatially-resolved investigation of CO2 methanation over Ni/γ-Al2O3 and Ni3.2Fe/γ-Al2O3 catalysts in a packed-bed reactor

CO2 methanation via the Sabatier reaction with (green) H2 is promising due to its role in achieving a carbon–neutral energy balance in the context of Power-to-Gas technologies. Since Ni-based catalysts are relatively inexpensive compared to other metals and exhibit high catalytic activity, they are most commonly used. Due to the exothermic nature of the reaction, strong temperature and concentration gradients occur, which influence the catalyst structure. Thus, revealing the effects of structural changes of the catalyst along the reactor bed on local activity and selectivity is essential. A 1D packed-bed reactor model was used for numerical simulations, coupled with detailed microkinetics and mass transport limitations. The simulation results are compared with axially-resolved concentration and temperature profiles over 17 wt% Ni/γ-Al2O3 and 17 wt% Ni3.2Fe/γ-Al2O3 catalysts at oven temperatures of 623 K and 723 K. Using additional information from structural spatially-resolved synchrotron-based operando X-ray absorption spectroscopy studies, the oxidation state of Ni was considered in modeling the reactor by changing the catalytically active surface area along the reactor. Predicted surface coverages are compared with surface species experimentally determined by diffuse reflectance infrared Fourier transform spectroscopy. Overall, this study demonstrates the importance of combining modeling with spatially-resolved and temperature-dependent experiments to improve multiscale models and make predictions more accurate.

Coupling covariance matrix adaptation with continuum modeling for determination of kinetic parameters associated with electrochemical CO2 reduction

Coupling covariance matrix adaptation with continuum modeling for determination of kinetic parameters associated with electrochemical CO2 reduction

Characterization of Oxygen and Ion Mass Transport Resistance in Fuel Cell Catalyst Layers in Gas Diffusion Electrode Setups

Fuel cell catalyst layers contain an essential active catalyst, a support material for electron conductivity, ionomer for proton conductivity, and porosity for gas transport, which build up complex interfaces that determine the overall performance. Subtle variations in the processing of the catalyst layers can significantly alter the performance, which demands intensive research efforts, and requires considerable amount of time. In the last few years, gas diffusion electrode (GDE) half-cell setups have been introduced as a promising approach to speed up catalyst layer evaluation. Yet, advanced methods to thoroughly characterize transport phenomena within the catalyst layer have not been established for GDE half-cell setups. In the present work, we adapt electrochemical characterization methods, such as O 2 transport resistance and CO-displacement, which have been previously developed for single cell testing, to enable unique insights into catalyst layers’ structure-performance relationships with the GDE method. Utilizing a commercial Pt/Vulcan catalyst as a test system, we identify the cause of mass transport limitations due to different ionomer contents. We show that an intermediate I/C ratio of 0.70, which forms a thin layer of ionomer, leads to an optimal performance for the Vulcan carbon support, due to an optimal compromise between O2 and proton accessibility.

Recent advances in sub-stoichiometric TiO2 as reactive electrochemical membranes (REM) for bio-refractory pollutants removal: A critical review

Water stress must be addressed by developing cost-effective wastewater treatment technologies. Reactive electrochemical membranes (REM) have gained popularity in this scenario due to the benefits of both electrochemical advanced oxidation processes (EAOP) and membrane filtration processes (MFP) in reducing mass transport limitations and fouling issues. In this context, this review aims to consolidate recent advances in REM, focusing on sub-stoichiometric titanium oxide-based REM (TiSO-REM), mainly Ti4O7, the most promising ceramic material due to its high conductivity and ability to generate •OH. The primary fabrication methods for TiSO porous anodes are discussed, resulting in a significant reminder of their electrocatalytic, porosimetric and physicochemical intrinsic properties. In addition to the real advantages of using this material in flow-through configurations, which allow convection-enhanced mass transport and provide a high electrochemical surface area. The latest progresses in TiSO-REM are presented, with specific emphasis on understanding the main operating parameters in performance efficiency: the determinative key role among pollutant nature, matrix effect and anode in removal mechanisms, and where the anode is in relation to the cathode in the reactor design. These points are intended to identify process limitations and challenges to be overcome when implementing various reactor configurations until the scaling-up. All these aspects are reviewed critically in order to provide future research directions for the industrial advancement of this technique, which has proven to be more energy-efficient and therefore economically feasible than alternative EAOP.

Mass transport limitations in concentrated aqueous electrolyte solutions: Theoretical and experimental study of the hydrogen–bromine flow battery electrolyte

Modelling and simulation is a powerful tool to support the development of novel flow cells such as electrolysers and flow batteries. Electrolytes employed in such cells often consist of aqueous solutions of highly concentrated solutes at elevated temperatures. Such conditions pose numerous challenges in conventional model parametrisation because of non-ideal behaviour of the electrolytes. The aim of this work is to study mass transport of electroactive species in highly-concentrated media.We selected the hydrogen–bromine flow battery posolyte, HBr (aq) and Br2, as an exemplary flow battery electrolyte and we leveraged chronoamperometric techniques involving ultramicroelectrodes to study diffusion and migration of bromide and bromine at high concentration and temperature. We successfully simulated the current densities of HBr/Br2 redox reactions in solutions up to 8 mol L–1 using advanced mass transport theory which agreed well with the results obtained with ultramicroelectrodes.While uncharged species transport (Br2) can be credibly modelled using conventional theories such as Fick’s law, charged species (Br–) require special treatment as the diffusion coefficient vary with concentration up to 50 % with respect to the limiting value at infinite dilution. The transport of charged species without added supporting electrolyte occurs via both migration and diffusion and the contribution of migration current may be up to 50 % of the total current. At HBr concentration > 0.6 mol L–1 migration appears to be suppressed due to the “self-screening” effect of the electrolyte.Proper experimental electrolyte characterisation under operating conditions similar to the actual flow cell applications is indispensable to establish predictive models and digital twins of electrochemical devices. Straightforward transfer of concepts known in electro-analytical chemistry to flow cells modelling may lead to erroneous simulations or model overfitting.

Plasma-assisted degradation of a short-chain perfluoroalkyl substance (PFAS): Perfluorobutane sulfonate (PFBS)

This study investigates the degradation of perfluorobutane sulfonate (PFBS), a chemical compound belonging to a group of per- and polyfluoroalkyl substances (PFAS), by gas-phase electrical discharge plasma. Plasma alone was ineffective in degrading PFBS due to its poor hydrophobicity, which inhibited the compound from accumulating at the plasma-liquid interface, the region of chemical reactivity. To overcome bulk liquid mass transport limitations, a surfactant, hexadecyltrimethylammonium bromide (CTAB), was introduced to interact with and transport PFBS to the plasma-liquid interface. In the presence of CTAB, ∼99% of PFBS was removed from the bulk liquid and concentrated at the interface, where 67% of the concentrate was degraded and 43% of that amount was defluorinated within one hour. PFBS degradation was further improved by optimizing the surfactant concentration and dosage. Experiments with a range of cationic, non-ionic, and anionic surfactants revealed that the PFAS-CTAB binding mechanism is predominantly electrostatic. A mechanistic understanding of the PFAS-CTAB complex formation, its transport to and destruction at the interface is proposed, alongside the chemical degradation scheme, which includes the identified degradation byproducts. This study shows that surfactant-assisted plasma treatment is one of the most promising techniques for destroying short-chain PFAS in contaminated water.