Mass Transport Conditions Research Articles

Voltammetric examination of carbon and platinum nanoelectrodes under mixed diffusion – migration mass transport conditions

The majority of electroanalytical experiments are performed with supporting electrolytes to minimize the migration of electroactive ions and other unwanted effects of the low conductivity of the system. Microelectrodes and nanoelectrodes allow for voltammetric measurements in solutions of substantial resistivity, however, their application in non-supported electrolytes is vaguely studied in detail. This work demonstrates the employment of carbon and platinum nanoelectrodes in solutions of ionic and neutral ferrocene derivatives in the presence and absence of supporting electrolyte. The ferrocene derivatives of various charges served as redox probes for the studies of diffusion-migration transport to nanoelectrodes. The obtained results are confronted with the theoretical predictions derived for microelectrodes. An addition of a supporting electrolyte stabilizes the voltammetric signal. It makes the voltammetric waves at nanoelectrodes well developed irrespective of the type of the redox system examined. The results revealed that the migration of electroactive species to nanoelectrodes in the absence of a supporting electrolyte does not introduce significant changes to the voltammetric signal recorded with a supporting electrolyte as much as for bigger microelectrodes. In many cases, the complete elimination of supporting electrolyte led to the poor development of the voltammetric waves at nanoelectrodes, possibly due to the breakdown of the electroneutrality in the region adjacent to the nanoelectrode, since the electrical double layer for nanoelectrodes may occupy a significant fraction of the transport depletion layer. The uncertainty in the concentrations of ionic impurities (that determine the actual ionic strength) in the layer adjacent to the electrode is another factor that may produce unpredictable and, to some extent, random voltammetric behavior of charged redox probes at nanoelectrodes in the absence of supporting electrolyte.

Effect of microfluidic channel integration onto gold microelectrode on its redox electrochemistry.

Microfluidic systems have attracted significant interest in recent years as they are extensively employed in lab-on-chip and organ-on-chip research. Their combination with electrochemical platforms offers many advantages, promising a high potential for sensing applications, still the microfluidic-channel integration onto electrodes might induce challenges related to changes in signal-to-noise ratios and mass transport conditions. In this study, we investigated the effect of microfluidic channel integration in redox behavior of thermally deposited gold thin film microelectrodes by voltammetric (CV and SWV) electrochemical measurements. Using different dimensions of PDMS microfluidic channels (i.e. widths of 50, 100, 250, and 500 μm) and a constant electrode dimension (200 μm), we analyzed the relationship between altered electroactive area and electrochemical response against target redox molecules. The increases in electroactive area which were determined by the microfluidic channel sizes were in well-correlation with the obtained CV and SWV redox currents as expected. There was no significant decrease in signal-to-noise ratio in microchannel-integrated electrodes. AFM and SEM characterization demonstrated that thermally deposited thin film electrodes had significantly lower (approximately 25 fold) surface roughness in comparison to commercial screen-printed electrodes. Additionally, we have observed a clear microelectrode-to-macroelectrode transition, from hemispherical to linear (planar) diffusion in other terms, with the increasing channel size.

Voltammetric Kinetic Studies of Electrode Reactions: Guidelines for Detailed Understanding of Their Fundamentals.

Theoretical and practical foundations of basic electrochemical concepts of heterogeneous charge transfer reactions that underline electrochemical processes are presented for their detailed study by undergraduate and postgraduate students. Several simple methods for calculating key variables, such as the half-wave potential, limiting current, and those implied in the kinetics of the process, are explained, discussed, and put in practice through simulations making use of an Excel document. The current-potential response of electron transfer processes of any kinetics (i.e., any degree of reversibility) are deduced and compared for electrodes of different size, geometry, and dynamics, namely: static macroelectrodes in chronoamperometry and normal pulse voltammetry, and static ultramicroelectrodes and rotating disc electrodes in steady state voltammetry. In all cases, a universal, normalized current-potential response is obtained in the case of reversible (fast) electrode reactions, whereas this is not possible for nonreversible processes. For this last situation, different widely used protocols for the determination of the kinetic parameters (the mass-transport corrected Tafel analysis and the Koutecký-Levich plot) are deduced, proposing learning activities that highlight the foundations and limitations of such protocols, as well as the influence of the mass transport conditions. Discussions on the implementation of this framework and on the benefits and difficulties found are also presented.

Porous Magnéli phase obtained from 3D printing for efficient anodic oxidation process

3D printing was used for the first time for synthesis of a 3D TiOx electrode applied to the removal of organic compounds from water by electrooxidation process. After characterization of the synthetized material, effectiveness and reaction mechanisms of the electrode for water treatment was assessed under galvanostatic mode through (i) using probe molecules (terephthalic acid and oxalic acid) and paracetamol (PCT) as model compound (ii) monitoring the fate of degradation by-products and mineralization yields, and (iii) quenching experiments using ethanol to clarify oxidation mechanism. Results emphasized that the suitable porous structure of the 3D TiOx promoted both direct electron transfer and hydroxyl radical-mediated oxidation at the electrode surface. Compared to Ti/TiOx and BDD (boron-doped diamond) plates, 3D TiOx anode achieved faster degradation of PCT (3.3- and 1.7- times enhancement, respectively), lower accumulation of degradation by-products and higher mineralization yield (75% vs 15% and 66%, for Ti/TiOx and BDD anodes, respectively, after 2-h treatment, under similar operating conditions). This study highlights the potential of 3D printing for designing electrode materials with suitable porous structure able to favor mass transport conditions and consequently enhance the efficiency of electrochemical advanced oxidation processes.

Engineering the Electrochemical Reaction Microenvironment to Valorize Nitrate-Polluted Wastewaters

Anthropogenic activities have imbalanced the global nitrogen cycle since the large-scale implementation of Haber-Bosch ammonia manufacturing. The removal of reactive nitrogen species (all inorganic forms besides dinitrogen) has fallen far behind their production, leading to heavy environmental burdens and continuous losses from the nitrogen economy. As the most prevalent waterborne reactive nitrogen pollutant, excessive nitrate jeopardizes both human and ecosystem health. Rather than treat nitrate as a waste, we electrochemically reduce nitrate (NO3RR) to ammonia, a commonly used fertilizer and clean fuel. Electrochemical treatment can enable on-site, modular treatment powered by renewable energy. As an inner-sphere reaction involving multiple hydrogenation and electron-transfer steps, the activity and selectivity of electrochemical NO3RR to NH3 is strongly influenced by properties at the electrode-electrolyte interface. Properties of the electrocatalyst (e.g., chemical composition, crystal structure) are crucial to intrinsic reaction activity and selectivity, while electrolyte properties (e.g., pH, species concentrations) also exert substantial impacts. Together, the electrode and interfacial electrolyte are referred to as the reaction microenvironment. Notably, the reaction environment is dynamically evolving during the reaction process: the electrocatalyst structure can undergo reconstruction, and the interfacial electrolyte properties usually deviate from the initial and bulk electrolyte properties. Titanium (Ti) has been identified as a robust NO3RR electrocatalyst, with appreciable NH3 selectivity and is suitable for real wastewater treatment due to its abundance, moderate cost, and corrosion resistance. We found that NO3RR introduces significant surface reconstruction and forms titanium hydride (TiHx, 0 < x ≤ 2). With a combination of ex situ grazing-incidence X-ray diffraction (GIXRD) and X-ray absorption spectroscopy (XAS), we demonstrated near-surface TiH2 enrichment with increasing NO3RR applied potential and duration. This quantitative relationship facilitated electrochemical treatment of Ti to form TiH2/Ti electrodes for use in NO3RR, thereby decoupling hydride formation from NO3RR performance. A wide range of NO3RR activity and selectivity on TiH2/Ti electrodes between −0.4 and −1.0 VRHE was observed and analyzed with density functional theory calculations. On the electrolyte side, mass transport modifies the interfacial electrolyte properties and thus regulates NO3RR activity and selectivity. In a representative flow-cell configuration with a Ti electrode, we systematically controlled mass transport conditions and demonstrated their impacts on NO3RR performance. With continuum model simulation and in situ infrared absorption spectroscopy, we characterized the interfacial environment under varied mass transport conditions and elucidated the impacts of interfacial electrolyte properties on NO3RR activity and selectivity. We found that diffusion layer thickness and interfacial cation concentration govern NO3RR activity, while interfacial pH steers NO3RR selectivity. In summary, our study underscores the importance of understanding the reaction microenvironment to inform rational catalyst design and electrolyte engineering to advance NO3RR performance.

Internal Mass Transport Induced Voltage Losses during Water Electrolysis on Interconnected Nickel Nanowire Mesh Electrodes

Nickel nanowire mesh electrodes offer great potential for water electrolysis in alkaline environment, given their high internal surface area, mechanical strength, and alkaline resistance. Here we use a nickel nanowire mesh with a specific area of 26 m2/cm3, resulting in ~100-fold area enhancement for a 4µm thick nanowire mesh, compared to planar nickel. This highly benefits geometric current density, potentially into a regime where mass transport is the main limiting factor. Therefore, we performed a systematic study of electrode behaviour under various mass transport conditions to decouple kinetic and mass transfer related contributions to electrode voltage losses.An electrochemical flow cell was designed to measure the nanowire mesh electrode performance under optimal mass transport conditions. The cell provided electrolyte flow through the nanomesh, enabling convective supply of reagent into the nanopores as well as convective removal of reaction products. Facilitated by the inherent strength of the nanowire mesh, superficial flow velocities up to 1 cm/s could be obtained, with which geometric current densities as high as 320 mA/cm2 could be measured in the kinetically limited (flowrate independent) regime.Internal mass transport limitations were isolated by use of an inverted rotating disk electrode (iRDE). Koutecky-Levich analysis was performed to compensate for external mass transfer contributions on both nanowire mesh electrodes and planar electrodes. The nanowire mesh as iRDE resembles operating conditions in an electrolyzer, whereas the planar electrode provides a well-defined benchmark area for the electrode surface activity.For both the oxygen evolution reaction as the hydrogen evolution reaction, mass transport limitations were studies as function of current density and electrolyte (NaOH) concentration. Nanowire mesh electrode operation in the presence of internal mass transport limitations was compared to the non-limited condition and planar benchmark, from which the electrode effectiveness factor could be calculated. A 1-d simplified model of the electrode was used to explain the observed phenomena and to provide guidance for design optimization.

Catalytic electrodes for the oxygen reduction reaction based on co-doped (B-N, Si-N, S-N) carbon quantum dots and anion exchange ionomer

B-N-, Si-N, and S-N-co-doped carbon quantum dots (CQD) were prepared by hydrothermal synthesis from inexpensive and non-toxic precursors. They were analysed by Raman, FTIR, and XPS spectroscopies and used as oxygen reduction reaction (ORR) electrocatalysts. The electrodes were prepared by drop-casting a slurry containing poly(sulfone trimethylammonium) hydroxide (PSU-TMA) to enhance the hydroxide-ion transport near the catalytically active centers. Furthermore, polyaniline (PANI), reported to be catalytically active for the ORR, was added to some electrodes. A similar trend was observed for the electrode capacitance measured by cyclovoltammetry in the non-Faradaic region and impedance spectroscopy. The ORR kinetics studied by linear sweep voltammetry showed the lowest onset and half-wave potentials and the lowest Tafel slopes for the B-N-co-doped samples, although the B concentration is low (0.5%). Data with and without PANI are quite similar. The S-N- and Si-N co-doped samples are slightly less efficient. The stability test demonstrated a decrease by about 10% of the diffusion-limited current after 500 cycles, indicating a microstructural change impacting the mass transport conditions, but the onset potential is not modified. Altogether, the good electrocatalytic activity of B-N-co-doped CQD with a low boron concentration in presence of PSU-TMA indicated the high potential of these electrodes. The inexpensive synthesis of co-doped CQD from non-toxic precursors is an important advantage of these materials vs other carbon-doped electrocatalysts.

Electrolyte Effects on CO2 Electrochemical Reduction to CO.

ConspectusThe electrochemical reduction of CO2 (CO2RR) constitutes an alternative to fossil fuel-based technologies for the production of fuels and commodity chemicals. Yet the application of CO2RR electrolyzers is hampered by low energy and Faradaic efficiencies. Concomitant electrochemical reactions, like hydrogen evolution (HER), lower the selectivity, while the conversion of CO2 into (bi)carbonate through solution acid–base reactions induces an additional concentration overpotential. During CO2RR in aqueous media, the local pH becomes more alkaline than the bulk causing an additional consumption of CO2 by the homogeneous reactions. The latter effect, in combination with the low solubility of CO2 in aqueous electrolytes (33 mM), leads to a significant depletion in CO2 concentration at the electrode surface.The nature of the electrolyte, in terms of pH and cation identity, has recently emerged as an important factor to tune both the energy and Faradaic efficiency. In this Account, we summarize the recent advances in understanding electrolyte effects on CO2RR to CO in aqueous solutions, which is the first, and crucial, step to further reduced products. To compare literature findings in a meaningful way, we focus on results reported under well-defined mass transport conditions and using online analytical techniques. The discussion covers the molecular-level understanding of the effects of the proton donor, in terms of the suppression of the CO2 gradient vs enhancement of HER at a given mass transport rate and of the cation, which is crucial in enabling both CO2RR and HER. These mechanistic insights are then translated into possible implications for industrially relevant cell geometries and current densities.

The gas diffusion electrode setup as a testing platform for evaluating fuel cell catalysts: A comparative RDE‐GDE study

AbstractGas diffusion electrode (GDE) setups have been recently introduced as a new experimental approach to test the performance of fuel cell catalysts under high mass transport conditions, while maintaining the simplicity of rotating disk electrode (RDE) setups. In contrast to experimental RDE protocols, for investigations using GDE setups only few systematic studies have been performed. In literature, different GDE arrangements were demonstrated, for example, with and without an incorporated proton exchange membrane. Herein, we chose a membrane‐GDE approach for a comparative RDE–GDE study, where we investigate several commercial standard Pt/C fuel cell catalysts with respect to the oxygen reduction reaction (ORR). Our results demonstrate both the challenges and the strengths of the new fuel cell catalyst testing platform. We highlight the analysis and the optimization of catalyst film parameters. That is, instead of focusing on the intrinsic catalyst ORR activities that are typically derived in RDE investigations, we focus on parameters, such as the catalyst ink recipe, which can be optimized for an individual catalyst in a much simpler manner as compared to the elaborative membrane electrode assembly (MEA) testing. In particular, it is demonstrated that ∼50% improvement in ORR performance can be reached for a particular Pt/C catalyst by changing the Nafion content in the catalyst layer. The study therefore stresses the feasibility of the GDE approach used as an intermediate “testing step” between RDE and MEA tests when developing new fuel cell catalysts.

Tunable Microgel‐Templated Porogel (MTP) Bioink for 3D Bioprinting Applications (Adv. Healthcare Mater. 8/2022)

Porous Bioinks for Bioprinting In article number 2200027 by Liliang Ouyang, Molly M. Stevens, and co-workers, a microgel-templated porogel bioink strategy is introduced to present micropores (10–100 µm) in the cell-laden building blocks for 3D bioprinting applications. It mediates the viscoelastic properties of a series of existing hydrogels and enhances embedded cell activity via better mass transport conditions.

Nanostructured, Metal-Free Electrodes for the Oxygen Reduction Reaction Containing Nitrogen-Doped Carbon Quantum Dots and a Hydroxide Ion-Conducting Ionomer.

In this work, we studied the combination of nitrogen-doped carbon quantum dots (N-CQD), a hydroxide-ion conducting ionomer based on polysulfone (PSU) and polyaniline (PANI), to explore the complementary properties of these materials in high-performance nanostructured electrodes for the oxygen reduction reaction (ORR) in alkaline solution. N-CQD were made by hydrothermal synthesis from glucosamine hydrochloride (GAH) or glucosamine hydrochloride and N-Octylamine (GAH-Oct), and PSU were quaternized with trimethylamine (PSU-TMA). The nanocomposite electrodes were prepared on carbon paper by drop-casting. Furthermore, we succeeded in preparing PSU-TMA + PANI + GAH-Oct fibers by electrospinning. The capacitance of the electrodes was investigated by cyclic voltammetry and impedance spectroscopy, which gave similar trends. The ORR was investigated by linear sweep voltammetry at rotating disk electrode speeds between 250 and 2000 rpm in an oxygen-saturated 1 M KOH solution. Koutecky–Levich plots showed that four electrons were exchanged for nanocomposite electrodes containing CQD. The highest reduction currents were measured for the electrodes containing GAH-Oct. The Tafel plots gave the lowest slope and the most positive half-wave potential for PSU-TMA + PANI + GAH-Oct fibers. The best electrocatalytic activity of this electrode could be related to the high amount of graphitic nitrogen in GAH-Oct. Long-term cycling tests showed no significant modification of the onset potential, but a change of the current in the mass transport limited region, indicated the evolution of the microstructure of the nanocomposite ORR electrode modifying the mass transport conditions during the first 400 cycles before reaching stationary conditions. FTIR spectra were used to study possible electrode degradation after the ORR in 1 M KOH: the only change was due to the reaction of PANI emeraldine salt to emeraldine base, whereas the other constituents of the multiphase electrode did not show any degradation.

Critical Step Length as an Indicator of Surface Supersaturation during Crystal Growth from Solution.

The surface processes that control crystal growth from solution can be probed in real-time by in situ microscopy. However, when mass transport (partly) limits growth, the interfacial solution conditions are difficult to determine, precluding quantitative measurement. Here, we demonstrate the use of a thermodynamic feature of crystal surfaces—the critical step length—to convey the local supersaturation, allowing the surface-controlled kinetics to be obtained. Applying this method to atomic force microscopy measurements of calcite, which are shown to fall within the regime of mixed surface/transport control, unites calcite step velocities with the Kossel–Stranski model, resolves disparities between growth rates measured under different mass transport conditions, and reveals why the Gibbs–Thomson effect in calcite departs from classical theory. Our approach expands the scope of in situ microscopy by decoupling quantitative measurement from the influence of mass transport.

Toward Understanding the Utilization of Oxygen Reduction Electrocatalysts under High Mass Transport Conditions and High Overpotentials

Toward Understanding the Utilization of Oxygen Reduction Electrocatalysts under High Mass Transport Conditions and High Overpotentials

Combining Nanoconfinement in Ag Core/Porous Cu Shell Nanoparticles with Gas Diffusion Electrodes for Improved Electrocatalytic Carbon Dioxide Reduction.

Bimetallic silver‐copper electrocatalysts are promising materials for electrochemical CO2 reduction reaction (CO2RR) to fuels and multi‐carbon molecules. Here, we combine Ag core/porous Cu shell particles, which entrap reaction intermediates and thus facilitate the formation of C2+ products at low overpotentials, with gas diffusion electrodes (GDE). Mass transport plays a crucial role in the product selectivity in CO2RR. Conventional H‐cell configurations suffer from limited CO2 diffusion to the reaction zone, thus decreasing the rate of the CO2RR. In contrast, in the case of GDE‐based cells, the CO2RR takes place under enhanced mass transport conditions. Hence, investigation of the Ag core/porous Cu shell particles at the same potentials under different mass transport regimes reveals: (i) a variation of product distribution including C3 products, and (ii) a significant change in the local OH‐ activity under operation.

Cover Feature: Interfacial pH Measurements Using a Rotating Ring‐Disc Electrode with a Voltammetric pH Sensor (ChemElectroChem 1/2022)

The Cover Feature illustrates a Rotating Ring-Disc Electrode in contact with an electrolyte. In this work, this electrode is functionalized with a pH sensing monolayer and is used to measure the interfacial pH during hydrogen evolution under controlled mass transport conditions. More information can be found in the Article by M C. O. Monteiro, X. Liu et al.

Interfacial pH Measurements Using a Rotating Ring‐Disc Electrode with a Voltammetric pH Sensor

AbstractElectrochemical reactions in which H+ or OH− ions are produced or consumed, affect the pH near the electrode surface. Probing the pH locally is therefore highly desired to understand and model the reaction environment under operando conditions. We carried out interfacial pH measurements under mass transport control using a rotating ring‐disc electrode (RRDE) coupled with our recently developed voltammetric pH sensor. The interfacial disc pH is detected by functionalizing the gold ring with a hydroxylaminothiophenol (4‐HATP)/4‐nitrosothiophenol (4‐NSTP) redox couple. As protons only have to interact with a monolayer containing the 4‐HATP/4‐NSTP, the sensitivity and time resolution that can be achieved are superior to potentiometric sensors. We used hydrogen evolution as a model reaction and performed measurements in buffered and unbuffered electrolytes. The effects of the current density, potential, the buffer capacity of the electrolyte and rotation rate on the local pH were investigated. This work shows a reliable and sensitive method for accurately probing the reaction environment under well‐defined mass transport conditions, over a wide pH range.

Characterization of the Depth of Discharge-Dependent Charge Transfer Resistance of a Single LiFePO4 Particle.

The discharged state affects the charge transfer resistance of lithium-ion secondary batteries (LIBs), which is referred to as the depth of discharge (DOD). To understand the intrinsic charge/discharge property of LIBs, the DOD-dependent charge transfer resistance at the solid-liquid interface is required. However, in a general composite electrode, the conductive additive and organic polymeric binder are unevenly distributed, resulting in a complicated electron conduction/ion conduction path. As a result, estimating the DOD-dependent rate-determining factor of LIBs is difficult. In contrast, in micro/nanoscale electrochemical measurements, the primary or secondary particle is evaluated without using a conductive additive and providing an ideal mass transport condition. To control the DOD state of a single LiFePO4 active material and evaluate the DOD-dependent charge transfer kinetic parameters, we use scanning electrochemical cell microscopy (SECCM), which uses a micropipette to form an electrochemical cell on a sample surface. The difference in charge transfer resistance at the solid-liquid interface depending on the DOD state and electrolyte solution could be confirmed using SECCM.

Optimization of in Situ pH Detection

High selectivity and electrode efficiency are essential for practical CO2 reduction [1-2]. However, the simultaneous hydrogen evolution reaction in aqueous solutions lowers process efficiency. Both reactions change the local pH, which in turn, has been shown to have an impact on CO2 reduction reactions [2-5]. In situ detection of pH using ultramicroelectrodes can help elucidate the pH dependency of these reactions [5-6]. However, the presence of the ultramicroelectrode tip can significantly influence conditions near the surface due to transport restrictions. Using a numerical model that accounts for the presence of the tip, the operando environment near the surface can be elucidated.In this work, a 2D-axisymmetric finite element (FEM) simulation was developed to predict concentration fields of reacting species near the electrode surface and in the presence of the ultramicroeletrode tip. The simulation can be used to identify optimal geometric and spatial parameters for experimental surface characterization. Using the FEM model, the operando pH within 100 µm of the surface electrode varies by up to 6 pH units within a 0.5 - 0.9 V vs. Ag/AgCl potential window. This wide variation is due to a diffusion boundary layer that grows with decreasing potential. The near-surface concentration of buffer species (Figure 1) was found to vary, along with pH, as a function of electrode potential and time.Lee, C. W.; Cho, N. H.; Im, S. W.; Jee, M. S.; Hwang, Y. J.; Min, B. K.; Nam, K. T. New Challenges of Electrokinetic Studies in Investigating the Reaction Mechanism of Electrochemical CO2 Reduction. J. Mater. Chem. A 2018, 6 (29), 14043–14057. https://doi.org/10.1039/C8TA03480J.Goyal, A.; Marcandalli, G.; Mints, V. A.; Koper, M. T. M. Competition between CO2 Reduction and Hydrogen Evolution on a Gold Electrode under Well-Defined Mass Transport Conditions. J Am Chem Soc 2020, 142 (9), 4154–4161. https://doi.org/10/gg84dq.Zhang, B. A.; Ozel, T.; Elias, J. S.; Costentin, C.; Nocera, D. G. Interplay of Homogeneous Reactions, Mass Transport, and Kinetics in Determining Selectivity of the Reduction of CO2 on Gold Electrodes. ACS Cent. Sci. 2019, 5 (6), 1097–1105. https://doi.org/10/ggbvcs.Seifitokaldani, A.; Gabardo, C. M.; Burdyny, T.; Dinh, C.-T.; Edwards, J. P.; Kibria, M. G.; Bushuyev, O. S.; Kelley, S. O.; Sinton, D.; Sargent, E. H. Hydronium-Induced Switching between CO2 Electroreduction Pathways. J. Am. Chem. Soc. 2018, 140 (11), 3833–3837. https://doi.org/10.1021/jacs.7b13542.Monteiro, M. C. O.; Jacobse, L.; Touzalin, T.; Koper, M. T. M. Mediator-Free SECM for Probing the Diffusion Layer PH with Functionalized Gold Ultramicroelectrodes. Analytical Chemistry 2020, 92 (2), 2237–2243. https://doi.org/10/gg77vg.Carneiro-Neto, E. B.; Lopes, M. C.; Pereira, E. C. Simulation of Interfacial PH Changes during Hydrogen Evolution Reaction. Journal of Electroanalytical Chemistry 2016, 765, 92–99. https://doi.org/10/f8pbnv. Figure 1

Understanding the role of mass transport in tuning the hydrogen evolution kinetics on gold in alkaline media.

In this work, we present an in-depth study of the role of mass transport conditions in tuning the hydrogen evolution kinetics on gold by means of rotation rate control. Interestingly, we find that the hydrogen evolution reaction (HER) activity decreases with the increasing rotation rate of the electrode. As we increase the rotation (mass transport) rate, the locally generated hydroxyl ions (2H2O +2e- → H2 + 2OH-) are transported away from the electrode surface at an accelerated rate. This results in decreasing local pH and, because of the need to satisfy local electroneutrality, decreasing near-surface cation concentration. This decrease in the near-surface cation concentration results in the suppression of HER. This is because the cations near the surface play a central role in stabilizing the transition state for the rate determining Volmer step (*H-OHδ--cat+). Furthermore, we present a detailed analytical model that qualitatively captures the observed mass transport dependence of HER solely based on the principle of electroneutrality. Finally, we also correlate the cation identity dependence of HER on gold (Li+ < Na+ < K+) to the changes in the effective concentration of the cations in the double layer with the changes in their solvation energy.

Easy-to-Use Osmosis-Based Microfluidic Chip for Protein Crystallization: Application to a Monoclonal Antibody

We developed a disposable microfluidic chip which mimics the vapor diffusion method for exploring protein crystallization conditions but at the nanoliter scale. This device exploits the permeation of water through a thin poly(dimethylsiloxane) (PDMS) layer separating droplets stored in the chip and containing a mixture of proteins and precipitants from an open microfluidic reservoir. The water chemical activity fixed by the reservoir makes it possible to modify the volume of the droplets in a controlled way (reduction or increase). Because PDMS is only permeable to water, the imposed water activity therefore increases or decreases the concentration of the solutes present in the droplet and consequently the supersaturation of the solution. The specificity of our approach, in addition to the low volumes ensuring controlled mass transport conditions, is that the concentration of all the solutes is known at any time, thus allowing the extraction of quantitative information on the crystallization process. We exploit these chips on a protein of therapeutic interest: the full-length monoclonal antibody anti-CD20. Our experiments allow us in particular to estimate both the solubility of this protein and the width of the metastable zone with Na2SO4 and PEG400 as crystallizing agents. We also show that the fine-tuning of the permeation rate makes it possible to perform crystallization/dissolution cycles to selectively dissolve small crystals and increase the mean size of the remaining anti-CD20 crystals.