Three-dimensional Porous Electrode Research Articles

Hierarchical mesh-type oxygen electrode using carbon nanotube framework for anion-exchange membrane-based unitized regenerative fuel cells

Developing three-dimensional porous oxygen electrodes (OE) is crucial for enhancing round-trip efficiency (RTE) of anion-exchange membrane-based unitized regenerative fuel cells (AEM_URFCs). Herein, a novel OE design of AEM_URFCs is presented based on a hierarchical mesh electrode (HME) with a porous carbon nanotube (CNT) framework. The HME, featuring square-shaped macropores achieved by dispersing catalyst nanoparticles on the CNT framework enhances surface area, mass transport, and electron transport. Half-cell test demonstrated superior oxygen reduction reaction and oxygen evolution reaction activity compared to conventional electrodes. Moreover, the HME maintained stable ORR and OER activity during the stability test. In single-cell tests, the AEM_URFCs with the HME exhibited higher RTE (55% at 20 mA cm−2), surpassing the conventional electrode (52%). Furthermore, RTE was maintained at a remarkable 41% under ultrahigh current density (250 mA cm−2), which is unprecedented in the literature. The AEM_URFCs featuring the HME displayed superior cyclability over 5 cycles and durable performances under both fuel cell and water electrolysis modes.

Ti3C2Tx/laser-induced graphene-based micro-droplet electrochemical sensing platform for rapid and sensitive detection of benomyl

The design and fabrication of high-performance electrode devices are highly important for the practical application of electrochemical sensors. In this study, flexible three-dimensional porous graphene electrode devices were first facilely fabricated using common laser ablation technique at room temperature. After then, hydrophilic two-dimensional MXene (Ti3C2Tx) nanosheet was decorated on the surface of the laser-induced graphene (LIG), resulting in disposable Ti3C2Tx/LIG electrode devices. After introducing Ti3C2Tx nanosheet, the electrochemical active area, electron transfer ability of LIG electrode device and its adsorption efficiency toward organic pesticide benomyl was significantly boosted. As a result, the fabricated Ti3C2Tx/LIG electrode device exhibited significantly enhanced electrocatalytic activity toward benomyl oxidation. Based on this, a novel and ultra-sensitive electrochemical platform for micro-droplet detection of benomyl was achieved in the range of 10 nM–6000 nM with detection sensitivity of 169.9 μA μM−1 cm−2 and detection limit of 5.8 nM. Considering the low-cost Ti3C2Tx/LIG electrode devices are rarely used for electrochemical analysis, we believed this research work will contribute to exploring the broader application of MXene/LIG electrode devices in the field of electrochemical sensing.

Three-dimensional porous polyimide anode for high-rate lithium ion batteries

Polymeric electrode materials are widely used in many applications but still suffer from poor electrical conductivity and slow transport kinetics, especially poor performance at high current densities. Herein, a polyimide electrode structure with 3D-ordered interconnected gradient pores was developed to improve the lithium-ion transport efficiency. The homogeneous mixed PMTA-PMMA composite was prepared by a simple one-pot method, followed by removal of PMMA (polymethyl methacrylate) spherical porogenic agent during the synthesis of polyimide material at high temperatures to form a low curvature gradient-pore structure. The large pores in the electrode enhanced the ion transport ability and improved the electrolyte infiltration rate, while the small pores improved the active specific surface area and guided the deposition of lithium ions. As a result, a high specific capacity reaching 907 mAh/g was maintained even after 1000 cycles at 5 A/g, a value about 3-fold that of un-pored electrode materials. The experimental studies and DFT calculations were used to develop a 24Li storage mechanism of PMTA, and the active electrode material was fully electrochemically activated by the constructed three-dimensional porous electrode structure. Overall, the proposed porous material with a high active area and porosity looks potential for use as an ultra-fast charging and discharging electrode of lithium-ion batteries with high specific capacity and excellent cycling performance.

Insights into the hydrogen evolution reaction in vanadium redox flow batteries: A synchrotron radiation based X-ray imaging study

The parasitic hydrogen evolution reaction (HER) in the negative half-cell of vanadium redox flow batteries (VRFBs) causes severe efficiency losses. Thus, a deeper understanding of this process and the accompanying bubble formation is crucial. This benchmarking study locally analyzes the bubble distribution in thick, porous electrodes for the first time using deep learning-based image segmentation of synchrotron X-ray micro-tomograms. Each large three-dimensional data set was processed precisely in less than one minute while minimizing human errors and pointing out areas of increased HER activity in VRFBs. The study systematically varies the electrode potential and material, concluding that more negative electrode potentials of −200 mV vs. reversible hydrogen electrode (RHE) and lower cause more substantial bubble formation, resulting in bubble fractions of around 15%–20% in carbon felt electrodes. Contrarily, the bubble fractions stay only around 2% in an electrode combining carbon felt and carbon paper. The detected areas with high HER activity, such as the border subregion with more than 30% bubble fraction in carbon felt electrodes, the cutting edges, and preferential spots in the electrode bulk, are potential-independent and suggest that larger electrodes with a higher bulk-to-border ratio might reduce HER-related performance losses. The described combination of electrochemical measurements, local X-ray micro-tomography, AI-based segmentation, and 3D morphometric analysis is a powerful and novel approach for local bubble analysis in three-dimensional porous electrodes, providing an essential toolkit for a broad community working on bubble-generating electrochemical systems.

(Invited) Investigating the Influence of Electrolyte Additives and Anode Structures on Electroplating Performance Towards Use in Iron-Based Batteries

Environmental concerns and economic development motivate a shift towards an energy economy based on renewable energy sources. In this new paradigm, large scale energy storage plays a pivotal role in the integration of intermittent and fluctuating renewable energy technologies, such as wind or solar. Among the available solutions, redox flow batteries (RFBs) are promising contenders as they allow independent scaling of power and energy, have a projected long lifetime and lower costs than other battery technologies[1]. However, the most developed RFB systems to date, i.e. all vanadium-RFBs, are still not extensively deployed, due to the elevated and fluctuating costs of vanadium, and the geographical dependency on resource extraction. Iron-based flow batteries, on the contrary, rely on earth-abundant active materials and benign water as solvent, granting potential for cost-effective energy storage systems[2]. However, the efficiency of these electrochemical reactors is limited by challenges such as unoptimized electrolyte compositions, which cause parasitic generation of hydrogen gas or irregular coating distributions[3], and electrode materials that have not been optimized for this specific application. These can severely affect current density distribution, therefore conditioning the plating and stripping or detachment of plated regions (irreversible capacity loss), which limit battery efficiency and operational lifetime. Current cycle life of iron-based batteries has been reported around 2,000 while the US DOE establishes targets comprised between 1,000 to 10,000 cycles[4] Therefore, in this research we aim to understand the underlying mechanisms of iron electroplating and the key parameters which influence the resulting structures, to enhance overall iron-based battery performance.In this talk, we present our advances to improve iron-RFB efficiency and durability, focusing on the iron plating electrode. In this research, we first leverage planar, model iron surfaces in a Swagelok-type cell and employ a suite of electrochemical techniques (e.g., chronoamperometry, electrochemical impedance spectroscopy) to gain insight into the poorly understood process of iron electroplating. The experimental data is coupled with parameter fitting to pre-existing physical models[5]. Additionally, we study the influence of chelating agents, such as buffering agents, surfactants and levelers (e.g., ascorbate, tetrabutylammonium ion or thiourea, respectively), on plating phenomena and elucidate their effects on the kinetics and structure of the resulting iron coatings, as well as their cycling behavior in symmetric cell configuration. Overall, we find that buffering agents achieved the greatest improvement in terms of faradaic efficiency, resulting in moderate cell currents, while other additives, such as levelers, allow larger plating currents at the expense of limited faradaic efficiencies.In addition to electrolyte-induced effects, another bottleneck of iron-RFBs are the electrodes employed in the negative half-cell. Common electrodes rely on iron-containing films or sintered iron oxide particles to form 3D structures[6]. However, comprehensive analysis relating anode structure and composition to flow cell performance have not been addressed to date. Accordingly, we hereby study the effects of diverse types of negative electrodes. To minimize complexities emerging in porous electrodes, we first analyze the effects of electrode composition in 2D films (e.g., pure iron, doped iron or iron-carbon hybrid systems) on the cell performance via polarization. Building on these findings, and to further improve iron-RFB performance, we study the impact of electrode architecture, by introducing complex, three-dimensional porous electrodes that enable higher surface areas and mass transport rates. We find that these three-dimensional electrode structures with large pore sizes produce an enhancement in exploitable active species and increased capacity, although there arises a trade-off with the structural integrity of these 3D electrodes. With this research we hope to shed light onto the control of ion-ion, solid-liquid and solid-solid interplay for iron-based batteries to aid the design of tailored electrolytes and advanced electrodes to realize low-cost iron-based battery systems.

Preparation and performance of a three-dimensional porous copper-tin electrode for lithium-ion batteries

A 3D porous Cu/Sn/Cu6Sn5/Cu3Sn composite anode was successfully fabricated by electrodepositing tin on a copper foam foil and then conducting diffusion heat treatment at 200ºC for 3 h. The specific capacity of the electrode was 660.97 mAh/g at 0.2 C after 300 charge/discharge cycles. The high capacity and cycling stability of the electrode were due to the Cu-Sn phases and porous structure of the copper foam. The formation of the Cu6Sn5 and Cu3Sn phases decreased the tin content on the surface, reduced the volume expansion rate caused by lithiation/delithiation and effectively inhibited the generation of cracks and electrode pulverization. Furthermore, the number of active sites for the reaction between tin and lithium were significantly increased by the uniform distribution of tin on the surface of the copper foam and its large specific surface area.

Catalyst Application in Three-Dimensional Porous Electrodes for Alkaline Electrolysis

Hydrogen is critical in the green transition, and a key system for producing green hydrogen is through alkaline electrolysis. Although the alkaline electrolyzers are mature and commercially available, vast improvements are still expected in the future 1.Traditional and most commercially applied alkaline electrolyzer electrodes are made from massive nickel plates (or nickel-plated steel plates) with some scattered perforation. With the ongoing development of thin alkaline ion‐conducting membranes with low internal resistance, the benefit of three‐dimensional porous electrodes becomes obvious. With such electrodes, there will be no blind spots and the active catalytic area can be increased significantly. Today, most groups developing such 3D electrodes use commercial nickel foam as substrate. This limits the options to what is available in the market, moreover the purchase cost is high.The present work aims at developing high‐performing three‐dimensional hydrogen and oxygen evolution electrodes for alkaline electrolysis. The focus will be on the application of nanostructured electrocatalysts into three‐dimensional electrode structures, similar to nickel foams.The results obtained, so far, show homogeneous formation of nanoparticles on the surface. The analysed samples show mechanical flexibility as soon as the particles are deposited but samples become rigid after testing in a half cell setup with KOH 1 M electrolyte. Further analysis will explore changes to the microstructure. The roughness factor calculated by non-faradaic cyclic voltammetry is in the range of 75 cm2 ECSA / cm2geometric, showing higher value than the commercial Nickel foam.Initial experiments are carried out on as-prepared samples, but more advanced catalysts are also explored based on state-of-the-art materials. [1] IRENA - Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5 °C Climate Goal, International Renewable Energy Agency, Abu Dhabi (2020)

Multielectrode electrochemical cell for in situ structural characterization of amorphous thin-film catalysts using high-energy X-ray scattering

A multielectrode-based electrochemical cell allows the structural characterization of an amorphous thin-film water oxidation catalyst under various electrochemical potentials using high-energy X-ray scattering and atomic pair distribution function (PDF) techniques. A multielectrode with five electrodes provides a sufficiently low background signal to enable high-energy X-ray scattering (HEXS) measurements and amplifies the extremely low HEXS signals from samples for high-resolution PDF analysis of in situ data from thin-film catalysts. Glassy carbon (GC) creates a relatively low intensity HEXS pattern and is used as a working electrode. Instead of a three-dimensional (3D) porous electrode architecture, the flat geometry of the electrode enables various deposition techniques to be used for the preparation of a highly conductive metal oxide layer. PDF analysis demonstrates high spatial resolution for a 230 nm thick amorphous iridium oxide film deposited on two roughened 60 µm thick GC electrodes. The PDF analysis resolves the domain size and distinguishes changes in fine structure which are directly correlated with the structure and function of the catalysts. The results bring the opportunity to analyze the structure of nanometre-scale amorphous thin-film catalysts in an electrolyte-compatible and compact 3D-printed electrochemical cell in a three-electrode configuration.

In Situ Probing the Mass Transport Property Inside an Imitated Three-Dimensional Porous Bioelectrode.

Three-dimensional porous materials have been demonstrated as the most successful bioelectrodes in bioelectrochemical systems due to their high specific surface area and abundant adhesion regions for electroactive bacteria. However, the pore clogging potentially limits the mass transfer process inside the electrode due to the unreasonable structure design and long-term operation. The investigation of mass transport behavior in the porous scaffolds is of great significance for designing the electrode structure and optimizing bioelectrochemical system performance. To in situ characterize the mass transport behavior in the orderly pore structure, model electrodes with 100 copper wires (10 × 10) are constructed to imitate a three-dimensional porous structure (pore size: ∼150 μm) commonly employed in bioelectrodes. The poor proton effective diffusion coefficient solidly demonstrates that the mass transport inside the three-dimensional porous electrode is critically inhibited, leading not only to a progressive change and sparse biomass in the biofilm development process but also to biofilm acidification due to serious proton accumulation. It finally results in sluggish bacterial metabolic activity and a decreased electrocatalytic capacity. The interior space of porous electrodes cannot be adequately utilized, resulting in the inability to fully exploit the advantages of their abundant surface area. Consequently, the construction of gradient porous electrodes with small inner and large outer pores to enhance mass transport is a feasible proposal for enhancing performance. The proposed methodology of establishing model electrodes combined with the in situ detection technique within porous electrodes is crucial for acquiring various types of physicochemical information inside the bioelectrode, such as biofilm growth situation, biochemical reaction conditions, as well as mass transfer characteristics. More importantly, the work provides a fundamental basis for designing highly efficient bioelectrodes.

Well-assembled nanosheets of nickel-cobalt double hydroxide flower as a reversible faradic battery-type electrode material for high-performance hybrid supercapacitor

The design and advancement of the interconnected three-dimensional (3D) porous nanoarchitecture electrode materials with fascinating electrochemical performance are in high demand for high-performance hybrid supercapacitors (HSCs). Herein, we report a facile fabrication of 3D flower-like nickel-cobalt double hydroxides (NiCoDH) architecture via a template-free hydrothermal approach. The NiCoDH flower-like architecture is composed of self-assembling and interconnecting many thin nanosheets with thicknesses ranging from 11–16 nm. The open void space and vertically oriented nanosheets in the NiCoDH flower-like architecture could shorten the pathway for the charge and electrons transport and offer abundant electroactive sites for redox reactions to enhance the electrochemical performance of the electrode material. The flower-like NiCoDH electrode material reveals a high specific capacity (Qs) of 731 C g−1 at the current of 1 A g−1 and retains 461 C g−1 even at 20 A g−1, signifying the high rate capability of the obtained electrode material. The HSC is constructed by adopting the NiCoDH and activated carbon (AC) as the positrode and negatrode, respectively. Further, the HSC supplies a Qs of 352 C g−1, and it offers a high energy density of 58.30 Wh kg−1 at the power density of 21.09 kW kg−1 and an excellent long-term cyclic life (∼90.4% retention over 10,000 repeated GCD cycles).

Preparation and characterization of a flexible lithium ion electrode based on carbonized cotton fabric

In order to prepare flexible electrode with excellent performance, cotton fabric was used as the substrate in this study. The surface of cotton fabric was modified by polydopamine(PDA). The cotton fabric was then carbonized. Finally, a three-dimensional porous flexible electrode for carbonized cotton fabric was prepared. The maximum stress of the flexible electrode is 0.5 MPa. With the increase of carbonization temperature, the specific capacity of flexible electrode increases gradually. The specific capacity of flexible electrode reaches 282 mAh.g−1 at 0.1c charge/discharge rate. The specific capacity of flexible electrode reaches 200 mAh.g−1 at 5C charge/discharge rate. The specific capacity retention rate of the flexible electrode is above 95% when the cycle is 100 weeks at 0.1 c and 600 weeks at 5C. CV test results show that with the decrease of scanning rate, the more stable the cyclic performance of flexible electrode is, the better the reversibility of Li+ stripping/embedding is. The conductivity of the flexible electrode increases with the increase of carbonization temperature. The surface resistance and AC impedance of the flexible electrode are reduced. When the carbonization temperature is 1000°C, the surface resistance of the flexible electrode is 2 Ω.−1, and the AC impedance is 5 Ω.

Li Metal Storage in Porous Carbon Frameworks: Effect of Li–Substrate Interaction

Lithium-metal electrodes are of particular interest for next-generation rechargeable batteries because of their high specific capacity and low redox potential. Therefore, Li-metal-based batteries may afford higher energy densities than commercially available Li-ion batteries with graphite anodes. However, various bottlenecks have hampered the commercial development of these batteries, including uncontrolled Li dendrite formation and huge volume changes during cycling. Consequently, Li-metal batteries suffer from low Coulombic efficiency and poor cycling stability. In recent years, there has been extensive research on the design and construction of three-dimensional (3D) porous electrodes that can host metallic Li. However, the low pore utilization and uneven Li plating remain crucial issues. This can be understood in terms of electronic and ionic transport through the framework electrode. The carbon frameworks exhibit high electronic conductance; however, large resistance to Li+ migration in the electrolytes of internal and interparticle pores inhibits the penetration of Li+ deep into the electrode. In this work, we demonstrate that a strong interaction between Li and a lithiophilic nanolayer on a substrate plays a critical role in enhancing pore utilization in carbon framework electrodes. As a model architecture, we examine a Li storage process in a framework electrode consisting of porous carbon derived from metal-organic frameworks (MOFs) and a galvanically displaced Ag layer on a Cu substrate (Cu@Ag). The electrochemical experiments combined with operando XRD measurements and microstructural characterizations suggest that a lithiophilic Ag on the Cu substrate preferentially reacts with Li+ to form Li x Ag during the initial stage of Li plating. This Li x Ag phase acts as a seed that can regulate the subsequent Li plating, promoting confined Li storage in the carbon framework electrode while suppressing top plating. Because of these advantages, the MOF-C framework electrode on Cu@Ag exhibits better cycling stability (>250 cycles) than the MOF-C framework electrode on Cu (140cycles). However, when the thickness of the MOF-C framework is increased to 90 μm, the diffraction peak for Ag remains dominant throughout Li plating-stripping, and the formation of Li x Ag alloys is not clearly detectable in the diffraction patterns, suggesting that only a limited amount of Ag is involved in the alloying reaction with Li+. Based on the computational studies, the efficacy of lithiophilic layers toward improving pore utilization is discussed in terms of the kinetic competition between Li+ transport through porous channels and the interfacial reaction of Li+ with the substrate. This study conveys an important message that the Li-substrate interaction plays a vital role in promoting the confined Li storage; hence, it should be considered a key design factor for porous carbon frameworks with high capacity and long cycle lifetime.References Yun, H. R. Shin, E.-S. Won, H. C. Kang, J.-W. Lee, Confined Li metal storage in porous carbon frameworks promoted by strong Li-substrate interaction, Chem. Eng. J. 430 (2022) 132897.Jin, Y. Ye, Y. Niu, Y. Xu, H. Jin, J. Wang, Z. Sun, A. Cao, X. Wu, Y. Luo, H. Ji, L. J. Wan, Solid-solution-based metal alloy phase for highly reversible lithium metal anode, J. Am. Chem. Soc. 142 (2020) 8818–8826.Kim, J. Lee, J. Yun, S.H. Choi, S.A. Han, J. Moon, J.H. Kim, J.-W. Lee, M.-S. Park, Functionality of dual-phase lithium storage in a porous carbon host for lithium-metal anode, Adv. Funct. Mater. 30 (2020) 1910538.

A carbon nanotube@silicon-based three-dimensional porous photo-supercapacitor for self-powered UV detection

Miniaturized self-powered energy devices are indispensable units for next-generation portable electronics. Photo-supercapacitors (PSCs), as a reliable energy harvesting device, have been explored for self-powered applications by utilizing the photovoltaic effect of micro-supercapacitors. In this paper, we propose a self-powered PSC with bulk micro-machined three-dimensional (3D) porous asymmetric electrodes with a high-aspect ratio via deep reactive-ion etching (DRIE) technique for the detection of ultra-violet (UV) radiance. The as-fabricated porous CNT@TiC/CNT@TiO2 PSC delivers an areal capacitance of 2.69 mF/cm2 (@5 mV/s) and 7.35 mF/cm2 (@20 μA/cm2) before and after photo-charge process, respectively. The optical response of the PSC is linearly proportional to the UV power intensity with a photoresponsivity of 14.8 mA/W. A photocurrent density of 354 μA/cm2 and a photo-to-dark current ratio of 12 are experimentally obtained under a UV power intensity of 15 mW/cm2. The as-prepared PSCs are further integrated on-chip to implement a UV leakage warning system that can drive a white LED for more than 60 s under a UV power intensity of 10 mW/cm2, fully demonstrating the practical application of the 3D silicon-based device in UV monitoring systems.

Topology optimization for the design of porous electrodes

Porous electrodes are an integral part of many electrochemical devices since they have high porosity to maximize electrochemical transport and high surface area to maximize activity. Traditional porous electrode materials are typically homogeneous, stochastic collections of small-scale particles and offer few opportunities to engineer higher performance. Fortunately, recent breakthroughs in advanced and additive manufacturing are yielding new methods to structure and pattern porous electrodes across length scales. These architected electrodes are emerging as a promising new technology to continue to drive improvement; however, it is still unclear which structures to employ and few tools are available to guide their design. In this work we address this gap by applying topology optimization to the design of porous electrodes. We demonstrate our framework on two applications: a porous electrode driving a steady Faradaic reaction and a transiently operated electrode in a supercapacitor. We present computationally designed electrodes that minimize energy losses in a half-cell. For low-conductivity materials, the optimization algorithm creates electrode designs with a hierarchy of length scales. Further, the designed electrodes are found to outperform undesigned, homogeneous electrodes. Finally, we present three-dimensional porous electrode designs. We thus establish a topology optimization framework for designing porous electrodes.

Novel Design and Numerical Analysis of 3D Porous Electrodes for Micro-Supercapacitors

Micro-supercapacitors (MSCs), a clean and sustainable energy storage device, possess remarkable characteristics such as high energy/power autonomy, high charge-discharge cycles, and fast charge-discharge rates, making them a promising candidate for miniaturized energy storage components. However, the major constraint of MSCs is their low energy density. As a result, researchers have been working extensively to enhance the energy density of MSCs. This work presents a novel three-dimensional (3D) porous interdigitated MSC electrodes with parallel cylindrical holes. The proposed and popular octet structures were designed and compared using finite element analysis under uniaxial tensile loading. The proposed structure demonstrated more than 70% higher surface area, and since these holes are continuous along the electrode length, there will be no resistance to the ion movements. Because of the design's simplicity, it will be possible to produce 3D MSC electrodes with a larger surface area using unconventional manufacturing methods. The proposed structure will significantly improve the energy and power density of 3D MSCs.

A bi-functional Co–Ni layered double hydroxide three-dimensional porous array electrode derived from ZIF-L(Co)@ZIF-L(Co, Ni) for oxygen evolution reaction and supercapacitors

Developing high-efficiency bifunctional materials for electro-catalysis and supercapacitors are urgently needed but challenging. Herein, we develop a self-supporting Co–Ni LDH electrode prepared by in-situ growing ZIF-L(Co)@ZIF-L(Co, Ni) on carbon paper followed by a pseudomorphic transformation. The optimized Co–Ni LDH/carbon paper electrodes (CN-2/CP) exhibit excellent electrochemical activity and stability in oxygen evolution reactions (OER) and supercapacitors. The CN-2/CP electrode displays a low overpotential of 230 mV at 10 mA cm −2 and superior stability at 40-h chronopotentiometry for OER. For supercapacitor, the CN-2/CP electrode delivers a high specific capacitance of 1346 F g −1 at 1 A g −1 and maintains a capacitance of 88.5% after 7000 charge/discharge cycles at 20 A g −1 . Based on the physical and chemical characterization results, the high performance originates from the in-situ electrochemical conversion of metal hydroxide, improved conductivity, fast charge transfer at the interface and unique layered cross morphology providing more active sites. • In-situ synthesis of Co–Ni LDH 3D porous array on carbon paper. • The self-supported electrode exhibits high OER performance and stability. • The large surface area and high porosity of CoNi LDH provide abundant active sites.

Hybrid MoS2+x Nanosheet/Nanocarbon Heterostructures for Lithium-Ion Batteries

Modulation of electron/ion transport in electrodes through the appropriate mesoscale electrode structural design is essential to achieving effective utilization of nanoscale electroactive materials. Herein, nanosheet MoS2+x/carbon [one-dimensional (1D) carbon nanotube (CNT) or two-dimensional (2D) graphene nanoplatelet (GNP)] heterostructures are prepared via a simple, one-step hydrothermal method, resulting in high-loading (16.2–21.0 mg/cm2) binder-free three-dimensional (3D) porous electrodes. In lithium-based batteries, an anionic S22––S2– redox system is demonstrated based on combined structural characterization using X-ray photoelectron spectroscopy, Raman spectroscopy, and in situ synchrotron-based X-ray absorption spectroscopy to elucidate the electrochemical behavior of the Mo and S centers. MoS2+x-GNP electrodes delivered 177 mAh/g (2.9 mAh/cm2) in the first cycle and 78 mAh/g (1.3 mAh/cm2) after 100 cycles at a current of 3.2 mA/cm2, representing high capacities despite such a high material loading for a sulfur-equivalent system, with 44% capacity retention and good rate capability. Conversely, the MoS2+x-CNT heterostructure displayed lower capacity and more capacity fade at all rates, attributed to aggregation of the active and carbonaceous materials in these electrodes and poor access to MoS2+x edge sites, as visualized via 3D Raman mapping and electron microscopy. The significantly improved capacity retention of the MoS2+x-GNP system is attributed to the (i) morphology because the arrangement of the 2D MoS2+x nanosheets on the GNP substrate allows for edge sites with excess sulfur to be exposed, (ii) increased stability of the structure during cycling, and (iii) homogeneous dispersion of the active and carbonaceous materials, resulting in good electrical contact.

Numerical modeling of ion transport and adsorption in porous media: A pore-scale study for capacitive deionization desalination

A pore-scale model is presented to simulate the dynamic ion transport and adsorption processes in porous electrodes used for capacitive deionization (CDI). The Stokes equation governing water flow is solved using the lattice Boltzmann method and Nernst-Planck equation describing ion transport is solved using the finite volume method. The ion adsorption process is considered at the surface of carbon electrodes. After validation against analytical solutions and published results, the model is used to study the coupled water flow, ion transport and adsorption in both two-dimensional and three-dimensional porous CDI electrodes at the pore scale. The effect of electrode microstructure, electrical potential and flow velocity on the adsorption processes is quantitatively investigated, and the relative importance of various parameters is determined. The presented model can be a powerful numerical tool to quantitatively analyze ion transport and adsorption in porous electrodes, and may provide useful information for the design and optimization of CDI electrodes and operating conditions for desired desalination efficiency, water throughput and cost.

Out-of-plane faradaic ion concentration polarization: stable focusing of charged analytes at a three-dimensional porous electrode.

Ion concentration polarization (ICP) accomplishes preconcentration for bioanalysis by localized depletion of electrolyte ions, thereby generating a gradient in electric field strength that facilitates electrokinetic focusing of charged analytes by their electromigration against opposing fluid flow. Such ICP focusing has been shown to accomplish up to a million-fold enrichment of nucleic acids and proteins in single-stage preconcentrators. However, the rate at which the sample volume is swept is limited, requiring several hours to achieve these high enrichment factors. This limitation is caused by two factors. First, an ion depleted zone (IDZ) formed at a planar membrane or electrode may not extend across the full channel cross section under the flow rate employed for focusing, thereby allowing the analyte to "leak" past the IDZ. Second, within the IDZ, large fluid vortices lead to mixing, which decreases the efficiency of analyte enrichment and worsens with increased channel dimensions. Here, we address these challenges with faradaic ICP (fICP) at a three-dimensional (3D) electrode comprising metallic microbeads. This 3D-electrode distributes the IDZ, and therefore, the electric field gradient utilized for counter-flow focusing across the full height of the fluidic channel, and its large area, microstructured surface supports smaller vortices. An additional bed of insulating microbeads restricts flow patterns and supplies a large area for surface conduction of ions through the IDZ. Finally, the resistance of this secondary bed enhances focusing by locally strengthening sequestering forces. This easy-to-build platform lays a foundation for the integration of enrichment with user-defined packed bed and electrode materials.

Sono-Cavitation and Nebulization-Based Synthesis of Conjugated Microporous Polymers for Energy Storage Applications.

Conjugated microporous polymers (CMPs) are promising energy storage materials owing to their rigid and cross-linked microporous structures. However, the fabrication of nano- and microstructured CMP films for practical applications is currently limited by processing challenges. Herein, we report that combined sono-cavitation and nebulization synthesis (SNS) is an effective method for the synthesis of CMP films from a monomer precursor solution. Using the SNS, the scalable fabrication of microporous and redox-active CMP films can be achieved via the oxidative C-C coupling polymerization of the monomer precursor. Intriguingly, the ultrasonic frequency used during SNS strongly affects the synthesis of the CMP films, resulting in an approximately 30% improvement in reaction yields and ca. 1.3-1.7-times enhanced surface areas (336-542 m2/g) at a high ultrasonic frequency of 180 kHz compared to those at 120 kHz. Furthermore, we prepare highly conductive, three-dimensional porous electrodes [CMP/carbon nanotube (CNT)] by a layer-by-layer sequential deposition of CMP films and CNTs via SNS. Finally, an asymmetric supercapacitor comprising the CMP/CNT cathode and carbon anode shows a high specific capacitance of 477 F/g at 1 A/g with a wide working potential window (0-1.4 V) and robust cycling stability, exhibiting 94.4% retention after 10,000 cycles.