Mass Transport Effects Research Articles

Electrochemical CO2 Reduction at Silver from a Local Perspective

The electrochemical reduction of CO2 to chemical fuels and value-added chemicals is a viable pathway to store renewably generated electrical energy and to mitigate the negative impact of anthropogenic CO2 production. Herein, we study how the local reaction environment dictates the mechanism and kinetics of CO2 reduction to CO at an Ag electrode. The local reaction environment is determined using a hierarchical model that accounts for multistep reaction kinetics, specific surface charging state at a given electrode potential, and mass transport phenomena. The model reveals vital mechanistic insights into the reaction behavior. The increasing Tafel slope with overpotential is seen to be influenced by the surface charging relation and mass transport effects. In addition, the decrease of the CO current density at high overpotentials is found to be caused not only by the decrease in CO2 concentration due to mass transport, surface charge effects, and pH increase but also by lateral interactions between HCOOad, COOHad, and Had. Moreover, we explore how the electrolyte properties, including bicarbonate concentration, solvated cation size, and CO2 partial pressure, tune the local reaction environment.

Liquid metal enabled continuous flow reactor: A proof-of-concept

Liquid metal enabled continuous flow reactor: A proof-of-concept

Understanding the Effect of Feed Gas Humidity on the Freeze Start Behavior of Polymer Electrolyte Fuel Cells

Understanding the water dynamics during the sub-zero operation of polymer electrolyte fuel cells can help to optimize the materials and operating strategies to achieve successful freeze starts. This study employs sub-second X-ray tomographic microscopy to study the effect of downstream flow conditions on the water distributions during dynamic freeze starts from −30 °C by varying the feed gas humidity. An increase in the feed gas humidity resulted in a decrease in electrochemical performance. To probe the catalyst layer (CL)-micro-porous layer (MPL) interface, a MPL with grooves across it’s entire thickness was used. Imaging results showed that during the initial phase of performance drop due to ice formation, all the water produced was limited to the CL and membrane. The total water observed during the sub-zero operation in the MPL grooves was 1–2 orders of magnitude lower than the theoretical water produced and increased with an increase in the gas RH from 0% to 50% but reduced with further increase to 100% RH due to a lower cell performance. The overall saturation in the GDL was zero during the sub-zero operation and increased to less than 1% above 0 °C indicating minimal effect of the GDL mass transport on the freeze start performance.

The effect of Ekman mass transport and Ekman pumping velocity on the variability of sea surface temperature in the Arafura Sea

Arafura sea is located between the southern part of Papua Island and Aru Island. Previous studies on Sea Surface Temperature (SST) have described that the SST is strongly affected by the upwelling, but the effect of Ekman Mass Transport (EMT) and Ekman Pumping Velocity (EPV) has not yet been studied. In other areas, it has been shown that EMT and EPV generated by the winds could affect the SST. Thus, further research is needed to better understand the role of the winds on the variability of SST through the mechanism of EMT and EPV in the Arafura Sea. This study used SST data from a high-resolution satellite image (GHRSST) and wind data from a scatterometer satellite image (MetOp A ASCAT). The data were processed using the composite and time-series correlation. This study shows that the higher the wind speed, causes the colder the SST in the Arafura Sea. In contrast, when the wind speed is lower, the SST tends to be warmer. The variabilities of the SST are mostly related to the mixing process associated with the magnitude of EMT. In the shallow water where the calculated Ekman depth is deeper than the actual depth, EMT is more influencing than EPV. On the deeper water at the northeast of the Island of Aru, the negative EPV induces upwelling, bringing the colder water to the sea surface. Statistically, the correlation between EMT (EPV) and the SST in the shallow water of the Arafura Sea is considered strong (weak). On the other hand, at the deep water of the Arafura Sea (northeast of the Island of Aru) offers a strong correlation between the EPV and the SST, whereas the EMT and the SST correlation is considered weak.

Effect of Mass Transport on the Charge Storage Mechanism of Cobalt Hydroxide Nanosheets in an Aqueous Electrolyte

Although cobalt hydroxide (Co(OH)2) has received great attention as a layered electrode material for aqueous-based energy storage applications due to its high theoretical capacity, it is unstable and easily oxidized to be more stable phase in an aqueous medium. This leads to decrease in capacity and cycling performance, limiting its practical application. In addition, a wide range of electrolyte concentration (1-6M) has been utilized for Co(OH)2 electrode and the mechanisms related to charge storage performance were still unclear. To enhance the overall performance, a deep understanding of charge storage mechanisms of Co(OH)2 corresponding to mass transport and electrolyte concentration is needed. Herein, the hydrodynamic voltammetry experiment using rotating disk electrode (RDE) was performed to study the effect of mass transport of the ions in electrolytes on the charge storage mechanism of the as-electrodeposited cobalt hydroxide nanosheets in various concentrations of an aqueous potassium hydroxide (KOH) solution. The cycling performance related to rotation rates and electrolyte concentrations was also studied. Additionally, Operando X-ray absorption spectroscopy (XAS) was carried out to investigate the change in oxidation state of Co(OH)2 in different concentrations of KOH during charge/discharge processes. The set of information in this work will pay the way for further development of the cobalt-based energy storage technologies.

Visualizing the Spatial Heterogeneity of Electron Transfer on a Metallic Nanoplate Prism.

The involvement between electron transfer (ET) and catalytic reaction at the electrocatalyst surface makes the electrochemical process challenging to understand and control. Even ET process, a primary step, is still ambiguous because it is unclear how the ET process is related to the nanostructured electrocatalyst. Herein, locally enhanced ET current dominated by mass transport effect at corner and edge sites bounded by {111} facets on single Au triangular nanoplates was clearly imaged. After decoupling mass transport effect, the ET rate constant of corner sites was measured to be about 2-fold that of basal {111} plane. Further, we demonstrated that spatial heterogeneity of local inner potential differences of Au nanoplates/solution interfaces plays a key role in the ET process, supported by the linear correlation between the logarithm of rate constants and the potential differences of different sites. These results provide direct images for heterogeneous ET, which helps to understand and control the nanoscopic electrochemical process and electrode design.

Plaster ventilated façade system for renovating modern and ancient buildings. A CFD analysis

In the past decades several researches have been related to energy saving and emissions reduction. The technologies that exploit passive natural ventilation, like ventilated roofs and façades, have been recognized as the effective methods to provide energy saving and comfort. Ventilated façades represent dry assembled coating systems for buildings, traditionally made of panels in different materials. Very few ventilated façade systems with plaster finishes are already present on the building market. The potentiality of their design solution can be considered, e.g., when an historic building has to be recovered, since they can enhance the energy efficiency of building without changing its appearance. CFD simulations have been carried out by the authors in order to analyze the thermal energy behavior of a ventilated façade system with a plaster finishing and for comparing the benefits derived from its use to the corresponding unvented insulated façade. The ventilated façade shows a relevant energy saving thanks to the effect of ventilation: a reduction of 70% of heat flux was achieved, furthermore, a reverse conductance calculation showed relevant differences with the same calculated by thermo-physical material properties, since in this last calculation the heat and mass transport effect are not considered.

Experimental Validation of Fuel Cell Powered Energy Efficient Gallium Nitride Multilevel Inverter for Industrial Applications

This paper presents the air breathing fuel cell (ABFC) power system and its investigation for industrial applications. The ABFC actual behaviour is realized through the development of an empirical model by considering activation polarisation, ohmic polarisation and mass transport effects. The developed stack model feeds the power conditioning unit of the proposed power system. This paper will throw light on the usage of GaN wide bandgap (WBG) material based power semiconductor devices (PSDs) that are used to build 5-level cascaded H- bridge multilevel inverter and a boost converter. This in turn enhances the energy efficiency of the entire FC power system. These WBG PSDs present the phenomenal properties such as high saturated drift velocity, wide bandgap and high critical breakdown field. This multilevel inverter is implemented for facilitating a quality AC output besides offering the voltage control. The investigation is carried out with the development of power conditioning unit hardware setup besides the fabricated FC module for the validation the developed ABFC model.

Effect of Nonuniform Mass Transport on Nanobubble Nucleation at Individual Pt Nanoparticles

Effect of Nonuniform Mass Transport on Nanobubble Nucleation at Individual Pt Nanoparticles

Modelado de la isomerización de glucosa en un reactor de lecho empacado utilizando un nuevo enfoque del mecanismo de Briggs-Haldane

Nowadays, immobilized enzymes are utilized in several food industry applications. Some researches use apparent kinetic parameters in immobilized enzyme reactor systems, which are limited to such case studies. To enhance productivity in a packed bed reactor (PBR), a clear description of all the mechanisms (kinetic, intra-particle diffusive mass transport, fluid-particle convective mass transport, and axial dispersion) affecting the process should be established. The objective of this study was to model the isomerization of glucose in a PBR with calcium alginate beads (CAB), using an approach where the kinetic and diffusional mechanisms are described independently. The convective mass transfer (kL) and axial dispersion (Dz) coefficients were calculated from correlations. Validation was performed comparing predictions against experimental data (R2 = 0.907) of glucose conversion at the reactor’s outlet once steady state was reached. Under the study conditions, in contrast to the effect exerted by diffusive mass transport on fructose specific productivity, the effect of axial dispersion and convective mass transport is negligible. Analyzing different operation parameters via simulation, the particle size had the highest impact on the glucose bioconversion. By reducing the CAB size, the surface area is increased and thus the conversion. It is recommended to test new immobilizing agents or decreasing the CAB size, monitoring that immobilizing support preserves its stability and functionality.

Switching the O–O bond-formation mechanism by controlling water activity

Switching the O–O bond-formation mechanism by controlling water activity

Self-supported amorphous nickel-iron phosphorusoxides hollow spheres on Ni-Fe foam for highly efficient overall water splitting

The development of high-efficiency and economical bifunctional electrocatalysts in hydrogen evolution reactions (HER) and oxygen evolution reactions (OER) plays a key role in overall water splitting. In this study, a success was achieved in synthesizing the hollow nanospheres with amorphous nickel-iron phosphorusoxides nanoflakes layer grown on Ni-Fe foam (NiFe-POx/NFF). Firstly, with the assistance of oxygen bubbles templates, the hollow spheres comprised of NiFe-OH nanosheets were produced through the in-situ chemical oxidation of NFF surface. After phosphorylation, crystalline NiFe-OH nanosheets were transformed into amorphous NiFe-POx with plenty of defects and disorders, thus improving the density of active sites. The catalytic performance of NiFe-POx/NFF was further improved by a combination of stable 3D hierarchical nanostructure, effective mass transport and charge transfer, as well as the electrochemical specific surface area expanded by fluffy featherlike nanoflakes. The self-supported electrode of NiFe-POx/NFF exhibited outstanding catalytic property in the presence of alkaline electrolyte, including a small overpotential of 110 and 247 mV required to achieve 50 mA cm−2 for HER and OER in 1.0 M KOH aqueous solution, respectively. As a bifunctional electrocatalyst, NiFe-POx/NFF could provide a low cell voltage of 1.52 V to reach 20 mV cm−2 under a two-electrode system as required for the occurrence of overall water electrolysis under alkaline medium, while showing extended durability under continuous electrolysis without degradation.

Method—Practices and Pitfalls in Voltage Breakdown Analysis of Electrochemical Energy-Conversion Systems

Many electrochemical energy-conversion systems are evaluated by polarization curves, which report the cell voltage across a range of current densities and are a global measure of operation and state of health. Mathematical models can be used to deconstruct the measured overall voltage and identify and quantify the voltage-loss sources, such as kinetic, ohmic, and mass-transport effects. These results elucidate the best pathways for improved performance. In this work, we discuss several voltage-breakdown methods and provide examples across different low-temperature, membrane-based electrochemical systems including electrolyzers, fuel cells, and related electrochemical energy-conversion devices. We present best practices to guide experimentalists and theorists in polarization-curve breakdown analysis.

Enhancing the biogas production and the treated effluent quality via an alternative Palm Oil Mill Effluent (POME) treatment process: Integration of thermal pretreatment and dewatering

To address the limitations associated with the disposal management and the treatment process of POME, an alternative process that consists of eliminating the cooling ponds and integrating a pretreatment technology combined with a dewatering device was proposed. Since the proposed treatment process has not been tested previously, the objective of this study is to investigate the effect of thermal pretreatment and solid loading alteration via dewatering on a thermophilic continuous set up consisting of an upflow anaerobic sludge blanket – hollow centered packed bed (UASB-HCPB) bioreactor. It was established that the best performing solid loading was the 40S:60L, which is made up of 40% by volume of POME settled solids and 60% of clarified liquid. Thermally pretreated 40S:60L solid loading had a significantly enhanced biogas production, methane purity, chemical oxygen demand (COD), biological oxygen demand (BOD), and total suspended solids (TSS) removal efficiencies. The reactor exhibited stability owing to the low volatile fatty acid/alkalinity ratio obtained throughout the run. With a reactor capacity of 5L, thermally pretreated 40S:60L achieved a daily biogas production volume of 48L and a methane purity of 77.2%. The COD, BOD, and TSS removal efficiencies of the thermally pretreated 40S:60L solid loading were evaluated to be 97.5, 95.1, and 94.5%. The enhanced process performance is due to the consistent solids concentration, which allowed a more effective mass transfer and transport in the medium. Meanwhile, thermal pretreatment promoted cell lysis, making the intercellular organic matter readily available for the bacteria consortium.

Pulse Electrodeposition of Copper in the Presence of a Corrosion Reaction

This study has examined the pulse electrodeposition of copper from a deep eutectic solvent. Pulse plating parameters were selected using the methodologies originally developed for aqueous solutions, including constraints arising from double layer charging and mass transport effects. Although copper could be deposited under nearly all conditions, in many instances only partial plating was observed and current efficiencies were low. This effect was greatest at low duty cycles and/or when the off-time was long. This phenomenon was attributed to a corrosion (comproportionation) reaction occurring in the off-time. A simple model based on the corrosion reaction being under cathodic mass transport control was developed. While this could explain the main effects, it predicted a corrosion rate that was typically higher than observed. A variety of other models were then explored, the most plausible one indicating that the corrosion rate in the off-time is being controlled by a slow chemical dissolution step. This explained the main observations and predicts that, only for long pulse periods when steady-state conditions are approached, is the corrosion rate under cathodic mass transport control. Finally, it was found that benzotriazole could be employed to substantially reduce the corrosion rate and allow higher current efficiencies to be obtained.

Pulsed Current Plating Does Not Improve Microscale Current Distribution Uniformity Compared to Direct Current Plating at Equivalent Time-Averaged Plating Rates

Suppression of surface roughness and dendrite growth under pulsed current (p.c.) plating is a widely reported effect for a variety of electrodeposited metals. Often, this effect is attributed to the modulation of mass transport during pulsing. In the present contribution, we use numerical simulations and scaling analysis to shed light on the transient mass transport effects near a 2D microscale pattern subjected to p.c. plating. Specifically, we compare the microscale current distribution during p.c. to that during direct current (d.c.) plating at an equivalent time-averaged plating rate. Modeling shows that the more uniform current distribution for a given time-averaged plating rate is that obtained during d.c. plating. The current distribution during p.c. plating is found to be less uniform in comparison to d.c., and the mechanistic rationale underlying this effect is explained using scaling analysis. Results reported herein have implications to the understanding of pulsed currents in applications ranging from thin-film electroplating to battery charging.

(Keynote) Electrolyte and Mass Transport Effects in CO2 Electroreduction

This talk will discuss how this double layer structure, or more specifically cations in the electrochemical double layer, may affect the kinetics of electrocatalytic H2 evolution and CO2 reduction, both the activity itself ànd the competition. We will discuss both neutral and acidic electrolyte conditions, and will derive new design rules for optimal Faradaic efficiency.

Structural dependence of effective mass transport properties in lithium battery electrodes

Scanning electrochemical microscopy (SECM) has recently been reported as a convenient technique to characterize the apparent diffusion coefficients in porous media (D⋆) such as lithium-ion battery electrodes. The technique relies on hemispherical diffusion at the probe in the presence and absence of porous media. However, transition towards cylindrical diffusion follows as the diffusion layer becomes comparable to the media thickness, gradually limiting direct correlation between D⋆ and the SECM probe steady state current. In this work, we examine the experimental parameters that define this transitional frontier using simulations. A best practice thickness criterium is derived that ensures that the measured steady state current is ≳ 90% of that found in the ideal hemispherical case. Below this thickness limit the response was found to follow an empirical formulation with only two variables. Fundamentally, this quantifies diffusion in the transition regime between hemispherical and cylindrical transport. Practically, this correction formulation may be included into standard steady-state current expression to account for the deviation from hemispherical transport in “thin” electrodes. The analysis consolidates SECM as a reliable tool to characterize liquid phase mass transport within porous films over wide range of thicknesses, including films supported on blocking substrates, like lithium-ion battery electrodes.

Structuring of Si into Multiple Scales by Metal-Assisted Chemical Etching.

Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

A Parallel Coupled Lattice Boltzmann-Volume of Fluid Framework for Modeling Porous Media Evolution.

In this paper, we present a framework for the modeling and simulation of a subset of physical/chemical processes occurring on different spatial and temporal scales in porous materials. In order to improve our understanding of such processes on multiple spatio-temporal scales, small-scale simulations of transport and reaction are of vital importance. Due to the geometric complexity of the pore space and the need to consider a representative elementary volume, such simulations require substantial numerical resolutions, leading to potentially huge computation times. An efficient parallelization of such numerical methods is thus vital to obtain results in acceptable wall-clock time. The goal of this paper was to improve available approaches based on lattice Boltzmann methods (LBMs) to reliably and accurately predict the combined effects of mass transport and reaction in porous media. To this end, we relied on the factorized central moment LBM as a second-order accurate approach for modeling transport. In order to include morphological changes due to the dissolution of the solid phase, the volume of fluid method with the piece-wise linear interface construction algorithm was employed. These developments are being integrated into the LBM research code VirtualFluids. After the validation of the analytic test cases, we present an application of diffusion-controlled dissolution for a pore space obtained from computer tomography (CT) scans.