Nanoporous Carbon Scaffold Research Articles

Re(bipyridine)-Polypyrrole Monolayer Deposition on Nanoporous Carbon Scaffolds for Electrochemical CO2 Reduction

The CO2 concentration in the atmosphere has increased from 280 ppm in 1750 to nearly 420 ppm in 2022, leading to multiple environmental issues. Thus, developing an efficient approach for the conversion and use of CO2 is one of our most pressing objectives today. Among different strategies to remove CO2, the electrochemical conversion of CO2 to carbon-based fuels is actively being pursued by many researchers. However, CO2 is thermodynamically quite stable (the dissociation energy of a C=O bond is approximately 750 kJ mol-1) and thus the activation of the bonds in CO2 requires a considerable overpotential, especially when carried out at low temperatures in aqueous solutions, resulting in low efficiency. Although a range of gaseous and liquid products, including formic acid, carbon monoxide, methane, ethylene, and ethanol, can be produced by transferring multiple protons and electrons, the formation of CO is simpler and more straightforward for industrial adoption. In this process, renewable energy would be used to operate the CO2 electrolyzer to convert CO2 to CO, which can then be utilized as a chemical feedstock to produce a variety of fuels [1].Molecular catalysts have been widely studied for CO2 reduction, with research on the development of solid hybrid systems that comprise an immobilized catalyst on a support well underway. This method offers further advantages, as it can further facilitate catalyst recyclability and product separation. Carbon-based materials are the most extensively employed supports discussed in the literature, due to their low cost, high electrical conductivity, and flexibility. [Re(2,2′-bipyridine)(CO)3Cl] ([Re(bpy)]), commonly known as Lehn's catalyst, is among the most extensively studied molecular electrocatalysts for the selective CO2 reduction to CO. While various studies have focused on its optimization, showing excellent performance in organic solutions, CO2 reduction should ultimately be accomplished in aqueous media. Despite the fact that hybrid systems eliminate the requirement for aqueous solubility of the catalyst, examples of immobilized [Re(bpy)] for the stable and effective CO2-to-CO conversion in water remain uncommon.In our previous work, the [Re(4-(4-aminophenethyl)-4′-methyl-2,2′-bipyridine)(CO)3Cl] ([Re-(NH2-bpy)]) complex was synthesized and covalently anchored to a colloid-imprinted carbon (CIC) powder by the oxidation of its amino group. The CICs have a large surface area, homogeneous and controllable pore sizes, and ultra-low tortuosity. While the Re/CIC catalyst converted CO2 to CO with a high selectivity of 93% and a Re-based TON of around 900, its stability was poor and the coverage of the CIC by the Re catalyst was quite low [2]. In later work, catalyst anchoring on the CIC surface was achieved by electrochemical polymerization of the pyrrole group attached to the complex, producing uniform films that exhibited Faradaic efficiencies from 90-100% as well as very good long-term stability under catalytic conditions, producing CO with a selectivity of over 70% for at least 24 hours [3].Herein, we report a similar [Re(bpy)] hybrid electrode constructed by covalent surface tethering of the Re catalyst via pyrrole polymerization but using a self-supported nanoporous carbon scaffold. The advantages of adopting the NCS as opposed to powdered supports include the lack of needed binder, which can cover active sites, the ease of imaging the catalyst before and after testing, and the ability to carry out flow through experiments to overcome mass transport issues and also local pH effects. The CO2RR tests were performed in an H-cell using a KHCO3 solution saturated with CO2, while the catalyst was loaded onto the NCS either by immersing the NCS in a 5 mM Re solution followed by electrodeposition in a Re-free solution (Method A), or by electrodeposition at slow sweep rates in the Re-containing solution (Method B). The bare NCS was studied first, showing the exclusive production of hydrogen, similar to the CIC powders, whereas our hybrid electrode generated 100% CO between -0.4 and -0.7 V vs. RHE. Method A showed that the number of immersion steps used for Re deposition correlated with the coverage of the NCS by the catalyst, in turn giving higher selectivity, higher activity, and a significantly higher TOF (0.13 s-1 vs. 0.03 s-1, found with the CICs). This is due to better [Re] utilisation within the NCS pores (more electroactive [Re]) and a better morphology, retaining the open pores after catalyst deposition. However, the durability was compromised at higher CO2RR rates. In comparison, Method B showed similar excellent performance metrics, while also exhibiting better reproducibility for samples prepared identically. References Jin, S., et al., Angewandte Chemie, 2021. 133(38)Willkomm, J., et al., ACS Applied Energy Materials, 2019. 2(4)Willkomm, J., et al., ACS Catalysis, 2021. 11(3)

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.

N-Doped Nanoporous Carbon Scaffolds As Catalysts for CO2 Reduction

The need for clean energy solutions in order to significantly lower our carbon footprint is placing increasing attention on renewable energy, such as wind and solar. However, these technologies suffer from intermittency issues, thus making efficient energy storage and conversion a major requirement. One of the solutions to this problem is electrochemical CO2 reduction (CO2RR), using renewable energy to produce green fuels and chemicals. However, the reduction of CO2 is thermodynamically and kinetically unfavorable. To overcome this, the development of highly efficient and selective electrocatalysts is a key goal of CO2RR research.Carbon has been attracting interest as a promising candidate due to its high specific surface area, good conductivity, and low-cost. However, its catalytic performance is limited by the electroneutrality of the carbon atoms in the primarily graphitic lattice. In order to activate the CO2 molecules and enhance the adsorption of intermediates, a range of novel carbon catalysts has been developed, with nitrogen doping being the most common approach. The N atom has a similar size as C and thus the lattice mismatch after doping is minimal [1]. At the same time, the N atom also has a higher electronegativity than C which will break the electroneutrality of the carbon lattice and enhance the conductivity of the material [2-3]. Furthermore, N doping results in a range of N-based surface species, thus potentially allowing the tuning of the CO2RR products [4].In this work, a novel, self-supported, nanoporous carbon scaffold (NCS) was used as the carbon substrate. The NCS is a templated, binder-free mesoporous carbon material with tunable pore sizes, a high surface area, good conductivity, and scalability [5]. Here, N doping of the NCS was achieved by heat treatment in NH3. An in house flow cell that can be switched between flow-through and flow-by modes was developed, allowing the NCS to be used as a model material to understand the impact of fluid flow on mass transfer limitations in the CO2RR and on any local pH effects that may be present.The CO2RR performance was tested in the flow cell using a CO2-saturated KHCO3 solution. The bare NCS was confirmed to generated only H2, while the N-doped NCS gives roughly a 50:50 ratio of H2:CO at all potentials, with the onset potential being ca. -0.55 V vs RHE. The effect of the NCS pore size was also investigated, showing that an NCS membrane with a nominal 85 nm pore diameter produced roughly 45% CO, while the NCS-12 (12 nm pore size) material gave somewhat more CO (ca. 55-60%). At the same time, the NCS-12 gave lower current densities, despite its higher surface area, likely due to poorer pore accessibility. Furthermore, running the cell in the flow-through mode gave higher currents at all of the NCS-based catalysts, most likely due to removal of trapped gases. Current work is focused on determining the effect of the N content and the type of N-based functionalities attached to the NCS surface on the CO2RR performance.

Electrochemistry of new generation conformal polyaniline/carbon scaffolds with monodispersed nanopores and high capacitance

A fully conformal polyaniline film was electrodeposited within a nanoporous carbon scaffold (NCS), maintaining its 3D interconnected pores while increasing its capacitance, ideally suited for capacitive deionization and ultrafiltration applications.

Sandwich structure for enhancing the interface reaction of hexanitrohexaazaisowurtzitane and nanoporous carbon scaffolds film to improve the thermal decomposition performance

Improving the thermal decomposition performance of hexanitrohexaazaisowurtzitane (CL-20) by appropriate methods is helpful to promote the combustion performance of CL-20-based solid propellants. In this study, we synthesized a sandwich structure of CL-20 and nanoporous carbon scaffolds film (NCS) and emphatically studied the thermal decomposition performance of the composite structure. Thermogravimetric analysis and differential scanning calorimetry were used to measure the thermal decomposition process of the composite structure. The kinetic parameters of thermal decomposition were calculated by the thermal dynamic analysis software AKTS. These results showed that the thermal decomposition performance of the sandwich structure of CL-20 and NCS was better than CL-20. Among the tested samples, NCS with a pore size of 15 nm had the best catalytic activity for the thermal decomposition of CL-20. Moreover, the thermal decomposition curve of the composite structure at the heating rate of 1 K/min was deconvoluted by mathematical method to study the thermal decomposition process. And a possible catalytic mechanism was proposed. The excellent thermal decomposition performance is due to the sandwich structure enhances the interface reaction of CL-20 and NCS. This work may promote the extensive use of CL-20 in the field of solid rocket propellant.

Supercritical CO2 foaming strategy to fabricate nitrogen/oxygen co-doped bi-continuous nanoporous carbon scaffold for high-performance potassium-ion storage

Potassium-ion batteries (PIBs) have aroused enormous interest from researchers for future electrochemical energy storage due to the low cost and abundant reserve of potassium resource. Although hard carbon materials are regarded as promising candidates for PIBs anodes, it is still a great challenge to achieve a fast and stable charging/discharging process owing to the sluggish kinetics of large-radius potassium ions. Herein, a nitrogen/oxygen co-doped bi-continuous nanoporous carbon scaffold (denoted as BNCS) anode is successfully fabricated using low-cost polyacrylonitrile as precursor through a supercritical CO 2 foaming and pyrolysis method without any post-treatment. As an anode material for PIBs, the BNCS delivers a high reversible specific capacity (325 mA h g −1 at 0.1 A g −1 after 300 cycles), excellent rate performance (118 mA h g −1 at 10 A g −1 ) and ultra-stable cyclability (184 mA h g −1 at 1 A g −1 after 5000 cycles). The attractive electrochemical performances can be ascribed to the synergic contribution from hierarchically interconnected micro- meso -macroporous structure and nitrogen/oxygen co-doping. This template-free and scalable method provides a promising nanoporous carbon strategy for preparing high-performance PIBs anode materials. • A template-free supercritical CO 2 foaming and pyrolysis method is proposed. • A nitrogen/oxygen co-doped bi-continuous nanoporous carbon scaffold is fabricated. • The as-fabricated carbon is used as an anode for potassium-ion batteries. • The anode exhibits high capacity, great rate capability and long-cycle stability.

Spontaneous Imbibition Dynamics of Liquids in Partially-Wet Nanoporous Media: Experiment and Theory

Nanoscale spontaneous imbibition is a common process in nanoporous soil and unconventional reservoirs. Due to the complexity of these natural nanoporous media, the relevant spontaneous imbibition dynamics are still unclear. Thus, this paper studies spontaneous imbibition dynamics of liquids into a nanoporous carbon scaffold (NCS, with controllable wettability and pore geometry). The effects of evaporation and surfactant on spontaneous imbibition are also examined. For low-volatility liquids, a linear relationship between spontaneous imbibition height ( $$H$$ ) and square root of time ( $$\sqrt{t}$$ ) is observed. A modified Lucas–Washburn (L–W) equation is developed to describe the corresponding dynamics and to predict the NCS effective radius and the advancing water contact angle. A dimensionless time function is presented, which includes the properties of solid and liquid and their interactions, and can be used for upscaling spontaneous imbibition data in nanoporous media from laboratory to reservoir scales. Both a larger NCS pore diameter and pore throat diameter cause faster imbibition. The addition of surfactant into an aqueous solution increases the imbibition rate, although this influence is suppressed with pore size decrease. In contrast, for high-volatility liquids, a significant deviation from the linear relationship between $$H$$ and $$\sqrt{t}$$ is found at late times. The modified L–W equation is further developed to include evaporation effect. This study thus provides fundamental understanding of spontaneous imbibition dynamics at nanoscales. The developed theoretical models are expected to be applicable to important problems such as water infiltration into soil, fracturing fluid loss in unconventional reservoirs, and electrolyte migration in electrochemical devices.

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

HypothesisThe 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. ExperimentsA 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. FindingsThe 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.

A balance between catalysis and nanoconfinement towards enhanced hydrogen storage performance of NaAlH4

Owing to its favorable thermodynamics and high density, NaAlH4 has been widely regarded as a potential hydrogen storage material, but its practical application is hindered by the sluggish kinetics, high operating temperature and poor cycling stability. Here, taking advantage of Co-doped nanoporous carbon scaffolds as structural host, we develop a new strategy to balance the synergistic effect between the catalytic role of Co nanoparticles and the nanoconfinement role of porous carbon scaffolds via the controllable etching of Co nanoparticles towards enhanced hydrogen storage performance of NaAlH4. The etching of Co nanoparticles creates extra void spaces nearby catalytically active Co nanoparticles, which not only exerts the catalytic effect of Co nanoparticles, but also improves the nanoconfinement role in maintaining the cycling stability towards increased loading ratio and hence high systematic capacity. Induced by this balanced synergistic effect, the peak temperature for the dehydrogenation of NaAlH4 could be reduced to 164 °C, 97 °C lower than the bulk counterpart, even under an ultrahigh loading ratio of 67 %, and more importantly, the reversible systematic hydrogen storage capacity could still reach 3.3 wt.% after 5 cycles. This work opens up a new avenue to improve the hydrogen storage performance of various complex hydrides.

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).

N-Doped Nanoporous Carbon Scaffold As an Electrocatalyst for CO2 Reduction

The rising level of CO2 in the atmosphere poses a major threat to our global climate [1]. Renewable energy are promising alternatives but the utilization of renewable energy is challenging because of its intermittency. The key solution is to develop an energy storage system that can store energy and then release it as needed. CO2 reduction reaction (CO2RR) uses abundant CO2 present in the atmosphere and renewable energy as the input power. Therefore, increasing interest has been focused on electrochemical routes to transform CO2 into useful products. However, the reduction of CO2 is thermodynamically and kinetically unfavorable. To overcome the energy barrier of CO2RR, the development of high efficiency and high selectivity catalysts is a key goal of CO2RR research.Metallic catalysts have attracted much attention for CO2RR and have achieved some successes. However, most metallic catalysts exhibit large CO2RR overpotentials and insufficient selectivity. Also, the high price of noble metals is a key obstacle to scale-up and commercialization of these materials for CO2RR.Carbon is a very promising candidate to advance CO2RR due to its high specific surface areas and good conductivity. However, carbon atoms are electrically neutral and therefore it is difficult to activate the CO2 molecules and adsorb the intermediate. Therefore, it is necessary to develop novel carbon catalysts to enhance their catalytic activity for CO2RR. Nitrogen is the most commonly used carbon doping atom due to its high electronegativity, which leads to polarization of the adjacent carbon atoms, thus enhancing the electronic/ionic conductivity [2]. Many carbon materials, such as carbon nanotubes and graphene, have been doped with N and investigated as CO2RR catalysts [3][4], with some N-doped materials exhibiting a 85% Faradaic efficiency towards CO production[5].In this work, a nitrogen-doped templated nanoporous carbon scaffold (N-doped NCS) was investigated as a catalyst material for electrochemical CO2 reduction. The NCS is a novel, templated, binder-free, self-supported, fully tunable mesoporous carbon material[6] that gives a high active site density and good conductivity.NCS material, having a pore size of either 12, 50 or 85 nm, was heated in NH3 gas at 700 °C for 7 hours to prepare N-doped NCS. SEM and TEM were used to confirm the NCS morphology, while XPS, EDX and elemental analysis were used to determine the N content of the NCS material. The electrochemical performance of the N-doped NCS was carried out first using CV in CO2 sat. 0.1 M KHCO3 in a glass cell. After that, a membrane electrode assembly (MEA) CO2 electrolyzer was used to determine the CO2 reduction activity. An N-doped NCS (IrO2-coated) was used as the anode and an anion exchange membrane (AEM) was used as the separator during CO2 electrolysis, with humidified CO2 gas used at the cathode side. The gas products were collected from the cathode outlet and injected into a gas chromatography system for analysis. No liquid products were observed in the solution that was released to the cell outlet.The performance of N-doped NCS will be presented based on the results obtained in various solution-flow cell configurations. Effect of N-doped NCS preparation optimization will also be discussed. Based on both the CV results and the MEA CO2 electrolyzer data, the onset potential of CO2RR was comparable to what has been reported by others for N-doped carbons, but the high internal surface area of the NCS, combined with its high extent of N doping, may give the N-doped NCS some advantages. A maximum 90% FECO was achieved and the stability of the catalytic material was also studied in the flow cell systems.

Pt/Nanoporous Carbon Scaffold Catalyst Layer for Use in Symmetrical Proton Exchange Membrane Fuel Cells

Carbon microstructures are an essential component of both the cathode and anode catalyst layer in PEMFCs, typically serving as the catalyst support [1], and it is known that the carbon porosity, surface area, and its hydrophobicity/hydrophilicity have a significant effect on electrocatalytic activity and transport issues. This is due to the impact of the carbon on catalyst dispersion, Nafion distribution, electronic conduction, and mass transport limitations [2-4]. Typically, carbon black is used as the catalyst support because of its high surface area, low cost, good electrical conductivity [4,5], but it is susceptible to corrosion and does not pack uniformly, thus resulting in uncontrolled pathways for reactants/products through the catalyst layer [6,7]. Recently, there has been an increasing interest in using mesoporous carbons as catalyst supports for fuel cell application, due to the better accessibility of the internal carbon surfaces, and the tethered Pt nanoparticles, to the reactants, and better mass transport of reactants and products during fuel cell operation [8,9].Our team has recently developed a novel nanoporous carbon scaffold (NCS), which is self-supported, scalable, and highly tunable (its monodisperse pore size can be controllably varied from 10 to 100 nm), giving specific surface areas of 200 to 600 m2 g-1. These films are fully percolating, have very low tortuosity and a 90% porosity, good electronic conductivity of 2-10 S cm-1 and are robust [10]. While prior work has involved the preparation and testing of an MEA that contained Pt/NCS on the cathode side of the separator, in this work, the focus was on the first-time testing of a symmetrical cell with Pt/NCS on both sides of a Nafion membrane. Here, the Pt nanoparticles (NPs) were deposited throughout the NCS material by wet impregnation of the chloroplatinic acid precursor (Fig. 1) and then infiltrated with Nafion by drop-casting. The thickness of the Pt/NCS films used in the current study was 25 ± 2 μm, the NCS pore size was 85 nm, and the Pt loading at the cathode was < 1 mg Pt cm-2. The Nafion®117 membrane was sandwiched between two NCS catalyst layers by hot pressing and the performance of these new symmetrical MEAs was evaluated in a fuel cell test station using different temperatures. This presentation will focus on the performance of the symmetrical MEA design in comparison to conventional ink-deposited systems, as well as the durability of these novel materials. Because the NCS has such a uniform structure, discussion will also be focused on the changes observed to the carbon itself as well as to the Pt NP size and distribution after testing under PEMFC conditions.

(Invited) Self-Supported Nanoporous Carbon Scaffold As a Novel Material for Use in Fuel Cells and Electrolysis Cells

Several years ago, our group pioneered the use of mesoporous colloid imprinted carbon (CIC) powders (1-3) for a wide variety of electrochemical applications, including as catalyst supports in PEM fuel cells, as electrodes in supercapacitors and redox-flow batteries, sensors, and for water deionization. The CICs have an ordered, close-packed internal porous (spherical) structure and are typically prepared by imprinting a mesophase carbon pitch powder with a colloidal silica template, followed by carbonizing the imprinted pitch and then removal of the silica template. One of the biggest advantages associated with the CICs is the tunability of their pores, controlled by the size of the silica colloid (from ca. 10 to 100’s of nanometers) used in the templating step, with our CICs having pore sizes ranging from ca. 8 – 100 nm. This pore size is ideal for the deposition of catalytic nanoparticles inside the pores, as well as the infiltration of an ionomer phase, such as Nafion, for use in PEM fuel cell catalyst layers. However, one disadvantage of the CIC powders, especially in PEMFC applications, is that there are variable pore sizes between the CIC particles and thus varying access to Nafion and water. Also, the use of powders makes it challenging to image the same particle or region before and after catalyst testing to determine any changes that have occurred, including carbon corrosion.Partly for these reasons, as well as the desire for improved reproducibility of the catalyst layer microstructure, our team recently developed a novel, self-supported, nanoporous carbon scaffold (NCS) material (4,5) for use in PEMFCs, but also in electrolysis systems. The preparation method used to produce the NCS membranes is similar to what was used by our group to make the CIC powders, employing mesophase pitch as the carbon precursor and imprinting with silica colloids of controlled diameter. The pore size can be similarly tailored (from 5 nm to > 100 nm) and the thickness can be controlled to 1-100 microns, with the NCS being formed using a tape casting method. We have also successfully modified the internal wettability of the NCS through surface functionalization, examples being a hydrophilic group (e.g., -PhSO3H) or a hydrophobic group (e.g., PhF5), and have also successfully loaded the NCS with Pt nanoparticles using a variety of approaches. The electrochemistry of the NCS materials, carried out in aqueous solutions using 3-electrode circuitry, shows that the internal surface of the NCS is fully percolating, with all of the pores being 3-D interconnected. Similar to the CICs, the ink bottle-shaped pores within the NCS material are connected bypore necks, which are typically 25-50% of the diameter of the corresponding spherical pores. The necks may hinder mass transport within the NCS during device operation and thus efforts have been made to tailor the pore neck size. The performance of the NCS material within a full PEMFC MEA has also been determined as a function of NCS pore and pore neck size. As well, the behavior of the NCS material in a flow through mode, including for CO2reduction in an electrolysis cell, has been demonstrated, while also having been characterized using nitrogen gas sorption, field emission microscopy (FE-SEM), transmission electron microscopy (TEM), 3-D TEM, TEM tomography, and elemental analysis.

(Invited) Ordered, Nanoporous Carbon Scaffolds (NCS) for Use in Energy Conversion and Related Applications

Nanoporous carbon materials are of significant interest for use in various electrochemical applications, including as catalyst supports in fuel cells, sensors, and as electrodes in supercapacitors, redox flow batteries, and lithium batteries. The nanoporous carbons under study in our group include both soft- and hard-templated ordered mesoporous carbons as well as colloid-imprinted carbons (CICs). While all of these carbons have well-controlled and narrow pore size distributions and high surface areas, they are all produced in the form of powders, which can cause problems when used in electrochemical devices. This is due to challenges in relation to reproducible powder packing and binders must normally be used to hold particles together. The binder may then block ions and electrons from passing through the pores and between particles, thus preventing full utilization of their surfaces. Furthermore, fine particulates may also cause health concerns (e.g., PM2.5). In order to overcome these challenges, our research group has focussed on the development of a series of novel nanoporous carbon scaffolds (NCS membranes), possessing a fully ordered and monodisperse porous structure. The NCS materials have pore diameters in the range of 5 to 100’s of nm and contain a fully inter-connected nano/macro-porous network (porosity ~80%). These NCS membranes, which can be made to be 0.5 - 100’s of microns in thickness, also have a very good conductivity of 2-10 S/cm and are fully electrochemically accessible in aqueous solutions, exhibiting a capacitance of 40-100 F/g. The fabrication method used to form these novel NCS membranes is also environmentally friendly and has been demonstrated to be applicable for manufacturing at large scale. Due to these exceptional properties, the NCS materials are of great interest for use in a wide range of clean energy and water remediation products. In this presentation, the preparation and properties of these materials will be discussed, including the successful loading of the NCS membranes with catalytic metal nanoparticles and internal surface functionalization. An overview of the application of the NCS material in PEM fuel cells, redox flow batteries, capacitors, and in nanofiltration applications will also be presented.

Investigation of Nanoporous Carbon Scaffold with Ordered Pore Structure As Microporous Layer for PEM Fuel Cells

The microporous layer (MPL) is a key component of the membrane electrode assembly (MEA) of a polymer electrolyte membrane fuel cell (PEMFC). Historically, the MPL has been prepared by applying a slurry of carbon particles and Teflon onto the carbon paper. Together, the carbon paper with the MPL on it is known as the gas diffusion layer. The MPL fabricated by the aforementioned method results in a random pore structure varying both in pore size (ranging 20-200 nm) and wettability (hydrophobic and hydrophilic). Although the primary role of the MPL is thought to improve water management, delineation of the effect of pore size from wettability becomes difficult in a physically and chemically heterogenous porous structure of a conventional MPL. Recently, the Birss group has developed nanoporous carbon scaffolds (NCS) with an inverse opal structure that has well-defined and tunable pore sizes. The wettability of the internal surface of the NCS films can also be altered by functionalization, from highly hydrophilic to hydrophobic. We have successfully integrated the NCS as an MPL in PEMFCs and evaluated its performance under low and high RH operations. The effect of pore size and wettability of NCS-based MPLs on the performance of PEMFCs will be presented in this talk.

Anomalous H2 Desorption Rate of NaAlH4 Confined in Nitrogen-Doped Nanoporous Carbon Frameworks

Confining NaAlH4 in nanoporous carbon scaffolds is known to alter the sorption kinetics and/or pathways of the characteristic bulk hydride reactions through interaction with the framework at the interface, increased specific surface area of the resulting nanoparticles, decreased hydrogen diffusion distances, and prevention of phase segregation. Although the nanosize effects have been well studied, the influence of the carbon scaffold surface chemistry remains unclear. Here we compare the hydrogen sorption characteristics of NaAlH4 confined by melt infiltration in nitrogen-doped/undoped ordered nanoporous carbon of two different geometries. 23Na and 27Al MAS NMR, N2 sorption, and PXRD verify NaAlH4 was successfully confined and remains intact in the carbon nanopores after infiltration. Both the N-doped/undoped nanoconfined systems demonstrate improved reversibility in relation to the bulk hydride during hydrogen desorption/absorption cycling. Isothermal kinetic measurements indicate a lowering of the activ...

Electrode Materials for the All Vanadium Redox Flow Battery Using Mixed Acid Electrolyte

Operation of vanadium redox flow batteries at low (below 5◦C) and high (above 40◦C) temperatures presents a significant challenge for commercialization due to issues with the solubility of vanadium under these conditions. One strategy that has been developed is the use of a mixed sulfuric and hydrochloric chloride acid electrolyte to stabilize the vanadium species. In this study, the electrochemical characteristics of the mixed acid system was explored using cyclic voltammetry at low and high temperature, with a range of electrode materials, including carbon paper, graphite felt and nanoporous carbon scaffold (NCS). Scanning electron microscopy (SEM) was used to characterize the morphology of the electrodes materials. The results show that the mixed acid system stabilized the vanadium redox system and enable operation at higher concentrations, leading to significant improvement in electrochemical characteristics at low and high temperature. Also, NCS electrode materials were found to have higher activity than graphite paper and graphite felt for the vanadium redox systems in the mixed acid electrolyte.

Effect of Eutectic Melting, Reactive Hydride Composites, and Nanoconfinement on Decomposition and Reversibility of LiBH4–KBH4

Eutectic melting, reactive hydride composites, and nanoconfinement have the potential to improve the reversible hydrogen storage properties in metal borohydrides. We study and compare the combined effect of all three methods on reversible hydrogen release and uptake of the eutectic melting lithium potassium borohydride system, 0.725LiBH4–0.275KBH4, with low melting temperature (Tm = 105 °C). The eutectic mixture and reactive hydride composites of the eutectic mixture with Mg or MgH2 are melt infiltrated into a CO2 activated nanoporous carbon scaffold, and their properties are compared to those of bulk samples. The decomposition of 0.725LiBH4–0.275KBH4 and the reactive hydride composites initiates simultaneously with the melting at 105 °C, but the decomposition remains slow until higher temperatures are reached (T > 300 °C). Eutectic melting appears to improve kinetics of hydrogen release and absorption, while nanoconfinement lowers the main hydrogen release temperature in the first cycle by up to 200 °C. ...

Characterization of the Dehydrogenation Process of LiBH4 Confined in Nanoporous Carbon

Annealing in the transmission electron microscope allows direct comparison of the same nanoporous carbon scaffold particles following dehydrogenation of the confined LiBH4. At a nominal temperature of 200 °C, a granular crust of cubic and cuboidal LiH nanocrystals ∼8–15 nm wide grew on the outer surface of the scaffold. Furthermore, these crystals migrated on the carbon support film away from the scaffold. Ejection of material from the scaffold also occurred upon dehydrogenation in a differential scanning calorimeter. The form of the ejected material was different, being bristles instead of cubes or cuboids. This ejection of LiH and thus preferential segregation of lithium from boron is proposed to explain the observed continual decrease in hydrogen storage capacity with the number of cycles.