Hydrophobic nanoporous carbon scaffolds reveal the origin of polarity-dependent electrocapillary imbibition.

An engineered nanoporous carbon scaffold (NCS) consisting of a 3-D interconnected 85 nm nanopore network was used here as a model material to investigate the nanoscale transport of liquids as a function of the polarity and magnitude of an applied potential ('electro-imbibition'), all in 1 M KCl solution. A camera was used to track both meniscus formation and meniscus jump, front motion dynamics, and droplet expulsion, while also quantifying the electrocapillary imbibition height (H) as a function of the applied potential of the NCS material. Although no imbibition was seen over a wide range of potentials, at positive potentials (+1.2 V vs. the potential of zero charge (pzc)), imbibition was correlated with carbon surface electro-oxidation, as confirmed by both electrochemistry and post-imbibition surface analysis, with gas evolution (O2, CO2) seen visually only after imbibition was well underway. At negative potentials, vigorous hydrogen evolution reaction was observed at the NCS/KCl solution interface, well before imbibition began at -0.5 Vpzc, proposed to be nucleated by an electrical double layer charging-driven meniscus jump, followed by processes such as Marangoni flow, adsorption induced deformation, and hydrogen pressure driven flow. This study improves the understanding of electrocapillary imbibition at the nanoscale, being highly relevant in a wide range of multidisciplinary practical applications, including in energy storage and conversion devices, energy-efficient desalination, and electrical-integrated nanofluidics design.

Deciphering the Interaction of Single-Phase La0.3Sr0.7Fe0.7Cr0.3O3-δ with CO2/CO Environments for Application in Reversible Solid Oxide Cells.

A detailed study aimed at understanding and confirming the reported highly promising performance of a La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr) perovskite catalyst in CO2/CO mixtures, for use in reversible solid oxide fuel cells (RSOFCs), is reported in this work, with an emphasis on chemical and performance stability. This work includes an X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electrochemical study in a range of pO2 atmospheres (pure CO2, CO alone (balance N2), and a 90-70% CO2/10-30% CO containing mixture), related to the different conditions that could be encountered during CO2 reduction at the cathode. Powdered LSFCr remains structurally stable in 20-100% CO2 (balance N2, pO2 = 10-11-10-12 atm) without any decomposition. However, in 30% CO (balance N2, pO2 ∼ 10-26 atm), a Ruddlesden-Popper phase, Fe nanoparticles, and potentially some coke are observed to form at 800 °C. However, this can be reversed and the original perovskite can be recovered by heat treatment in air at 800 °C. While no evidence for coke formation is obtained in 90-70% CO2/10-30% CO (pO2 = 10-17-10-18 atm) mixtures at 800 °C, in 70 CO2/30 CO, minor impurities of SrCO3 and Fe nanoparticles were observed, with the latter potentially beneficial to the electrochemical activity of the perovskite. Consistent with prior work, symmetrical two-electrode full cells (LSFCr used at both electrodes), fed with the various CO2/CO gas mixtures at one electrode and air at the other, showed excellent electrochemical performance at 800 °C, both in the SOFC and in SOEC modes. Also, LSFCr exhibits excellent stability during CO2 electrolysis in medium-term potentiostatic tests in all gas mixtures, indicative of its excellent promise as an electrode material for use in symmetrical solid oxide cells.

Scalable nanoporous carbon films allow line-of-sight 3D atomic layer deposition of Pt: towards a new generation catalyst layer for PEM fuel cells.

Although nanoporous carbons are ubiquitous materials that are used in many clean energy and environmental applications, most are in powder form, thus requiring binders to hold particles together. This results in uncontrolled and complex pathways between particles, potentially exacerbating mass transport issues. To overcome these problems, we have developed an unprecedented binderless, self-supported, nanoporous carbon scaffold (NCS) with tunable and monodisperse pores (5-100+ nm), high surface area (ca. 200-575 m2 g-1), and 3-dimensional scalability (1-150+ cm2, 1-1000 µm thickness). Here, it is shown that NCS85 membranes (85 nm pores) are particularly promising as a host for the homogeneous and efficient 3-D atomic layer deposition (ALD) of Pt nanoparticles, due to the facile penetration of gas phase Pt precursor throughout the homogeneous, low tortuosity internal structure. Furthermore, the high density of surface defects of the as-synthesized NCS promotes uniform Pt nucleation with minimal agglomeration. These advantageous features are key to the rapid oxygen reduction kinetics observed under polymer electrolyte membrane (PEM) fuel cell MEA testing conditions. Cells constructed with an optimal ALD Pt loading of 30 cycles are shown to exhibit a specific activity of ≥0.4 mA cm-2Pt which is exemplary when compared to two commercial catalyst layers with comparable Pt mass loadings and tested under the same conditions. Furthermore, a maximum power density of 1230 mW cm-2 (IR-corrected) is obtained, with the limiting current densities approaching a very respectable 3 A cm-2.

Wetting dynamics of nanoliter water droplets in nanoporous media.

HYPOTHESIS: The Lucas-Washburn (L-W) equation is the classical theory to describe the dynamics of spontaneous imbibition in single micro-channels and micro-scale porous media. However, for nanoliter droplets imbibition in nanoporous media, the L-W equation may not be suitable, due to the nanoscale liquid-solid interactions, e.g., contact line pinning and capillary condensation. In addition, for an intrinsically hydrophobic nanoporous substrate, spontaneous imbibition of a nanoliter droplet is hypothesized to occur if capillary condensation had occurred internally already. EXPERIMENTS: A nanoporous carbon scaffold was synthesized and used as a model nanoporous medium. A recently-developed micro-injection technique was used to generate a series of nanoliter water droplets (2.8-34 nL); the entire wetting dynamics (i.e., apparent contact angle and droplet volume as a function of time) were observed inside an environmental scanning electron microscope. FINDINGS: The L-W equation does not describe the wetting dynamics of nanoliter water droplets in nanoporous media. A new theoretical model is developed to characterize the corresponding dynamics. It is demonstrated that, even for an intrinsically hydrophobic nanoporous substrate, spontaneous imbibition of a nanoliter droplet can occur if capillary condensation had occurred internally already.

Ultrasensitive and Label-Free Detection of the Measles Virus Using an N-Heterocyclic Carbene-Based Electrochemical Biosensor.

With the current intense need for rapid and accurate detection of viruses due to COVID-19, we report on a platform technology that is well suited for this purpose, using intact measles virus for a demonstration. Cases of infection due to the measles virus are rapidly increasing, yet current diagnostic tools used to monitor for the virus rely on slow (>1 h) technologies. Here, we demonstrate the first biosensor capable of detecting the measles virus in minutes with no preprocessing steps. The key sensing element is an electrode coated with a self-assembled monolayer containing the measles antibody, immobilized through an N-heterocyclic carbene (NHC). The intact virus is detected by changes in resistance, giving a linear response to 10-100 µg/mL of the intact measles virus without the need to label or process the sample. The limit of detection is 6 µg/mL, which is at the lower limit of concentrations that can cause infections in primates. The NHC-based biosensors are shown to be superior to thiol-based systems, producing an approximately 10× larger response and significantly greater stability toward repeated measurements and long-term storage. This NHC-based biosensor thus represents an important development for both the rapid detection of the measles virus and as a platform technology for the detection of other biological targets of interest.

Highly Ordered Nanoporous Carbon Scaffold with Controllable Wettability as the Microporous Layer for Fuel Cells.

We introduce a novel self-standing, nanoporous carbon scaffold (NCS, 25 µm thick), with an ordered inverse opal pore structure (∼85 nm pore) as a microporous layer (MPL) in a polymer electrolyte membrane fuel cell. Unlike previous studies, through chemical functionalization of the pore surfaces, the wettability of the MPL is controllably modified without altering the pore structure. Ex situ environmental scanning electron microscopy experiments revealed water sorption in the hydrophilic NCS under moderate relative humidity (RH) conditions but not in the hydrophobic NCS, wherein water condensation on the surface was noted only at high RH. The influence of structure and wettability of different MPLs on cell performance was gleaned from steady-state cell polarization behavior. For cells operated under dry conditions (≤80% RH), the limiting current for cells with a hydrophilic NCS MPL was the highest while that for cells with a hydrophobic NCS MPL was the lowest regardless of the level of water saturation (RH).

Organometallic-Derived Carbon (ODC)-Metal Nano-Oxide Composites as Improved Electrode Materials for Supercapacitors.

In the search for the new generation of electrochemical energy storage materials, a novel and straightforward synthetic route for porous carbons and metal oxide nanoparticle composites based on the chlorination of the organometallic compounds Ni(C5H5)2 and Mn(C5H7O2)2 at moderate temperatures, followed by hydrothermal treatment, has been developed. Electrochemical measurements in a three-electrode configuration show that, in both composites NiO@ODC and Mn3O4@ODC, a synergistic effect between the capacitive and pseudocapacitive energy storage mechanisms is observed, thereby improving their electrochemical performance vs pure carbon materials. Electrochemical evaluation of symmetric cells gave gravimetric capacitances of 124 and 130 F g-1 for NiO@ODC and Mn3O4@ODC, respectively. However, the porous structure of the carbon matrix and the higher conductivity of Mn3O4, together, were found to be responsible for the superior electrochemical performance of Mn3O4@ODC.

Enhanced Signal Amplification in a Toll-like Receptor-4 Biosensor Utilizing Ferrocene-Terminated Mixed Monolayers.

A major challenge in effectively treating infections is to provide timely diagnosis of a bacterial or viral agent. Current cell culture methods require >24 h to identify the cause of infection. The Toll-like Receptor (TLR) family of proteins can identify classes of pathogens and has been shown to work well in an impedance-based biosensor, where the protein is attached to an electrode via a self-assembled monolayer (SAM). While the sensitivity of these sensors has been good, they contain a high resistance (>1 kΩ) SAM, generating relatively small signals and requiring longer data collection, which is ill-suited to implementation outside of a laboratory. Here, we describe a novel approach to increase the signal magnitude and decrease the measurement time of a TLR-4 biosensor by inserting a redox-active ferrocenyl-terminated alkanethiol into a mixed SAM containing hydroxyl- and carboxyl-terminated alkanethiols. The SAM formation and modification was confirmed via contact angle and X-ray photoelectron spectroscopy measurements, with TLR-4 immobilization demonstrated through a modified immunosorbent assay. It is shown that these TLR-4 biosensors respond selectively to their intended target, Gram-negative bacteria at levels between 1 and 105 lysed cells/mL, while remaining insensitive to Gram-positive bacteria or viral particles at up to 105 particles/mL. Furthermore, the signal enhancement due to the addition of ferrocene decreased the measurement time to less than 1 min and has enabled this sensor to be used with an inexpensive, portable, hand-held potentiostat that could be easily implemented in field settings.

Nanoengineered Ircore@Ptshell Nanoparticles with Controlled Pt Shell Coverages for Direct Methanol Electro-Oxidation.

The design and application of bimetallic alloy nanoparticles (NPs) for electrocatalytic applications are challenged by the need to clearly identify and understand the individual effect of each component. In the present work, the focus has been on PtIr NPs, with alloyed NPs being previously shown to be active toward the methanol oxidation reaction (MOR), but for which the mode of action of the Ir component remains uncertain. We have therefore nanoengineered a family of Ircore@Ptshell NPs, using a modified polyol method, to control the Pt shell coverage (up to 2 monolayers) and thus to allow the separation of the bifunctional and electronic effects of Ir on the Pt activity. It is shown that the Ir core size and crystallinity do not change with the deposition of the Pt shell, as confirmed by transmission electron microscopy and X-ray diffraction. CO stripping and hydrogen underpotential deposition/removal were used for the first time to determine the surface composition of the Ircore@Ptshell NPs. It is shown that the Ircore enhances the MOR activity of the Ptshell primarily through the bifunctional effect, with an optimum Pt coverage of 0.4 of a monolayer. At 60 °C, an additional electronic effect of Ir on Pt can be discerned, causing an inhibition in the MOR rate by weakening the adsorption of methanol on the Ptshell, thus helping to remove the adsorbed CO intermediate from the shell surface.

Surface Characteristics of Microporous and Mesoporous Carbons Functionalized with Pentafluorophenyl Groups.

The in situ diazonium reduction reaction is a reliable and well-known approach for the surface modification of carbon materials for use in a range of applications, including in energy conversion, as chromatography supports, in sensors, etc. Here, this approach was used for the first time with mesoporous colloid-imprinted carbons (CICs), materials that contain ordered monodisperse pores (10-100 nm in diameter) and are inherently highly hydrophilic, using a common microporous carbon (Vulcan carbon (VC)), which is relatively more hydrophobic, for a comparison. The ultimate goal of this work was to modify the CIC wettability without altering its nanostructure and also to lower its susceptibility to oxidation, as required in fuel cell and battery electrodes, by the attachment of pentafluorophenyl (-PhF5) groups onto their surfaces. This was shown to be successful for the CIC, with the -PhF5 groups uniformly coating the inner pore walls at a surface coverage of ca. 90% and allowing full solution access to the mesopores, while the -PhF5 groups deposited only on the outer VC surface, likely blocking its micropores. Contact angle kinetics measurements showed enhanced hydrophobicity, as anticipated, for both the -PhF5 modified CIC and VC materials, even revealing superhydrophobicity at times for the CIC materials. In contrast, water vapor sorption and cyclic voltammetry suggested that the micropores remained hydrophilic, arising from the deposition of smaller N- and O-containing surface groups, caused by a side reaction during the in situ diazonium functionalization process.

Rapid and Low-Energy Fabrication of Symmetrical Solid Oxide Cells by Microwave Methods.

Solid oxide fuel cells are highly efficient, low-emission, and fuel-flexible energy conversion devices that can also run in reverse as solid oxide electrolysis cells, converting CO2 and/or H2O to useful fuels and pure O2. Our team has recently developed a highly promising mixed conducting oxide catalyst (La0.3Ca0.7Fe0.7Cr0.3O3-δ) that can be used at both the anode and cathode in either the fuel cell or electrolysis mode in a lower-cost symmetrical cell. However, there is still a need to improve material processing and cell manufacturing methods in this field. Here, we report, for the first time, fabrication of a symmetrical solid oxide cell, based on our very promising catalysts, using rapid, low-cost, low-energy, and green microwave (MW) processing techniques. These cells were fabricated with MW-sintered powders and were then MW-sintered without the use of any MW susceptors inside the electrode layers or any additional presintering steps. The catalyst layers show very stable nanostructures and do not delaminate, and the cells exhibit reaction rates that are similar to those obtained using normal ceramic processing methods. Importantly, the powder preparation and cell sintering steps, carried out using MW methods, require only ca. 1/3 and 1/9 of the time/energy, respectively, versus those required in traditional furnace methods, thus translating to significant cost savings.

Clarifying the role of Ru in methanol oxidation at Ru(core)@Pt(shell) nanoparticles.

The catalytic activity of Rucore@Ptshell nanoparticles (NPs) towards CO oxidation, a strongly adsorbed intermediate that compromises the performance of direct methanol fuel cells, is known to be significantly better than at Pt alone. However, a systematic study aimed at understanding the beneficial effect of Ru on Pt during the methanol oxidation reaction (MOR) has not been carried out as yet. Here, Rucore@Ptshell NPs, having a controlled Ptshell coverage of zero to two monolayers and two different Rucore sizes (2 and 3 nm), were synthesized using the simple polyol method to determine the precise role and impact of Ru on the MOR in 0.5 M H2SO4 + 1 M methanol at RT and 60 °C. Because the structure of our Rucore@Ptshell NPs is known with such certainty, we were able to show here that the rate of methanol adsorption/dehydrogenation can be accelerated either by compression of the Ptshell (by making the Rucore larger) when it is less than one monolayer in thickness, or by decreasing the electronic effect of the Rucore on the Ptshell (achieved by adding a second Pt layer to the Ptshell). At low overpotentials, decreasing the Ptshell thickness also helps in increasing the rate of the MOR by enhancing the rate of oxidation of adsorbed CO. Finally, it is shown that the bi-functional effect of Ru on the Ptshell plays only a minor role in the catalysis of the MOR, especially at large particles where CO surface diffusion is facilitated.

Electrochemistry of La(0.3)Sr(0.7)Fe(0.7)Cr(0.3)O(3-δ) as an oxygen and fuel electrode for RSOFCs.

The use of a single porous mixed ion-electron conducting (MIEC) material as both the oxygen and fuel electrodes in reversible solid oxide cells is of increasing interest, primarily due to the resulting simplified cell design and lower manufacturing costs. In this work, La(0.3)Sr(0.7)Fe(0.7)Cr(0.3)O(3-δ) (LSFCr-3) was studied in a 3-electrode half-cell configuration in air, pure CO2 and in a 1 : 1 CO2 : CO mixture, over a temperature range of 650-800 °C. A detailed analysis of the impedance (EIS) data, under both open circuit and polarized conditions, as well as the cyclic voltammetry response of LSFCr-3 has shown that it is very active in all of these environments, but with oxygen evolution being somewhat more facile that oxygen reduction, and CO2 reduction more active than CO oxidation. Evidence for a chemical capacitance, associated with the Fe(3+/4+) redox process in LSFCr-3, was also obtained from the EIS and CV data in all gas environments.

Gold nanoparticle array formation on dimpled Ta templates using pulsed laser-induced thin film dewetting.

Here we show that pulsed laser-induced dewetting (PLiD) of a thin Au metallic film on a nano-scale ordered dimpled tantalum (DT) surface results in the formation of a high quality Au nanoparticle (NP) array. In contrast to thermal dewetting, PLiD does not result in deformation of the substrate, even when the Au film is heated to above its melting point. PLiD causes local heating of only the metal film and thus thermal oxidation of the Ta substrate can be avoided, also because of the high vacuum (low pO2) environment employed. Therefore, this technique can potentially be used to fabricate NP arrays composed of high melting point metals, such as Pt, not previously possible using conventional thermal annealing methods. We also show that the Au NPs formed by PLiD are more spherical in shape than those formed by thermal dewetting, likely demonstrating a different dewetting mechanism in the two cases. As the metallic NPs formed on DT templates are electrochemically addressable, a longer-term objective of this work is to determine the effect of NP size and shape (formed by laser vs. thermal dewetting) on their electrocatalytic properties.

First-time electrical characterization of nanotubular ZrO2 films for micro-solid oxide fuel cell applications.

In this work, anodically grown ZrO2 nanotubes (NTs) are examined for the first time for use in micro solid oxide fuel cell (µ-SOFC) applications. This is due to their high surface area to volume ratio and useful nanoscale morphological features (e.g., 5-100 nm thick NT bases that could serve as the electrolyte layer). To understand their full potential for these applications, the determination of their electrical properties is necessary. Therefore, ZrO2 NTs, in the form of a uniform and crack-free film, were obtained by the two-step anodization of Zr foil in aqueous solutions. The films exhibited excellent adhesion to the Zr substrate, which facilitated impedance spectroscopy analyses, used for the first time to obtain the resistivity of the nanotubular array separately from the contact resistances. This gave a conductivity of the ZrO2 NTs of 1.6 × 10(-6) S cm(-1) at 600 °C in N2, approximately twice that reported for dense ZrO2 films measured at the same temperature in air, and also a very reasonable activation energy of 0.90 eV in the 400-600 °C temperature range.

Novel electrochemical fingerprinting methods for the precise determination of Pt(shell) coverage on Ru(core) nanoparticles.

The surface composition of nanoparticles is critical in defining their chemical and electrochemical properties. However, there are a limited number of tools that can rapidly and reliably establish these important characteristics at this small scale. In the present work, a series of Rucore@Ptshell nanoparticles (2 or 3 nm diameter Ru core, 0 to 2 monolayers of Pt in the shell layer) were synthesized and several novel electrochemical fingerprinting methods were developed to determine the Pt shell characteristics. These involved tracking the charge associated with the reduction of the oxide film formed on the exposed Rucore, as well as the potential and charge associated with COads stripping, giving the precise coverage of the first and second Pt monolayer, respectively.