Triple-phase Interface Research Articles

Designing Self-Supported Electrocatalysts for Electrochemical Water Splitting: Surface/Interface Engineering toward Enhanced Electrocatalytic Performance.

Electrochemical water splitting is regarded as the most attractive technique to store renewable electricity in the form of hydrogen fuel. However, the corresponding anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) remain challenging, which exhibit complex reactions and sluggish kinetic behaviors at the triple-phase interface. Material surface and interface engineering provide a feasible approach to improve catalytic activity. Besides, self-supported electrocatalysts have been proven to be highly efficient toward water splitting, because of the regulated catalyst/substrate interface. In this Review, the state-of-the-art achievements in self-supported electrocatalyst for HER/OER have demonstrated the feasibility of surface and interface engineering strategies to boost performance. The six key effective surface/interface engineering approaches for rational catalysts design are systematically reviewed, including defect engineering, morphology engineering, crystallographic tailoring, heterostructure design, catalyst/substrate interface engineering, and catalyst/electrolyte interface regulation. Finally, the challenges and opportunities on the valuable directions are proposed to inspire future investigation of highly active and durable HER/OER electrocatalysts.

A surface-supporting method for an anode material of Li4Ti5O12 via an epitaxial thin film approach

High-speed rechargeability is essential for next-generation secondary batteries. Introducing a surface-supporting material deposited on a cathode material accelerates Li-ion motions between an electrode−electrolyte interface by an electric field concentration at a supporting material-cathode-electrolyte (triple-phase) interface (TPI). In addition, a high relative permittivity material was found to be a promising supporting material with which to reinforce the electric field concentration at TPIs. However, the TPI’s effects on anode materials remains to be revealed. To demonstrate those effects, we prepared CeO2 or BaTiO3 micropads deposited on Li4Ti5O12 epitaxial thin films. Compared with the cathodes, CeO2 micropads deposited on Li4Ti5O12 film showed the best performance at a high C-rate. Because the rate-determining step of Li4Ti5O12 epitaxial thin films is inner diffusion, reinforcing the surface electric field by the deposition of a low relative permittivity materials could promote high C-rate performance even in anode materials.

Electrically conductive hydrophobic membrane cathode for membrane distillation with super anti-oil-fouling capability: Performance and mechanism

This study mainly focused on enhancing oil-membrane electrostatic repulsion towards anti-oil-fouling via electrically conductive hydrophobic membrane in electricity-assisted membrane distillation (MD). Carbon nanotubes (CNTs) were coated on commercial membranes to fabricate membrane cathode. For concentrated hexadecane-in-saline water emulsions, the modified membrane exhibited significantly less flux decline, < 5% in dealing with extreme high-concentration oil emulsion (2000 ppm) at cell potential of 3.0 V. The anti-oil-fouling robustness was further confirmed over a continuous three-cycle operation with in-situ DI rinse. The anti-fouling mechanism was systematically discussed regarding the hydrophilicity of the membrane interface, charge repulsion between the oil and membrane cathode as well as slippery property at the liquid-gas-solid triple-phase interface. A modeled fouling rate constant was negatively associated with the calculated capacitive surface charge of the membrane cathode. Thermodynamics analysis suggested enhanced foulant-membrane electrostatic repulsion leads to a significant energy barrier which favored anti-fouling performance. Interestingly, the sliding dynamics of oil droplet along the interface of membrane cathode was found, tuned by the herein weak cell potential, which could also contribute to the fouling mitigation. Our results insight that the electrically conductive membrane cathode could modulate the foulant-membrane interaction which plays an important role in mitigating fouling/wetting occurred at the triple-phase interface.

Utilizing the different distribution habit of La and Zr in Li-rich Mn-based cathode to achieve fast lithium-ion diffusion kinetics

The commercialization prospect of Li-rich Mn-based (LRM) cathode materials lies in their high energy density (>900 Wh kg−1), but the practical application in many scenarios is hindered by their low intrinsic ionic conductivity. Herein, we increase the ionic conductivity and cycling performance of LRM cathode materials by utilizing the different distribution habit of La3+ and Zr4+. La atoms tend to accumulate on the grain surface, while Zr4+ is likely to be doped into the bulk of LRM cathode, which is confirmed by the theoretical calculation and experimental results. The surface is modified by the island-shaped and conductive LaMnO3+δ (LMO) and La2Zr2O7 (LZO) compounds, constructing a triple-phase interface (TPI) for the rapid lithium-ion diffusion. Meanwhile, the bulk LRM lattices are doped by Zr4+ to stabilize the layered framwork. The modified LRM cathode calcined at 650 °C exhbits a high specific capacity of 192.6 mAh g−1 after 200 cycles at 2C rate with superior Li+ diffusion coefficients and enhanced rate capability. This study sheds light on how to rationally improving the ionic conductivity of LRM cathode for its practical application.

Homogeneous triple-phase interfaces enabling one-pot route to metal compound/carbon composites

Metal compounds (e.g., metal phosphides/sulfides/selenides) coupled with carbon materials have recently drawn great attraction for boosting the electrochemical performances because of their appealing synergistic effect and valuable structural stability. Despite many examples for their synthesis exist, there is still a need for a simplistic and comprehensive approach to such metal compound/carbon (MC/C) composites. Herein, an effective, facile, yet versatile strategy to produce various types of MC/C composites is presented. Key to this strategy is construction of a homogeneous triple-phase interface, which is realized by utilization of a hybrid assembly integrated with carbon, metal and sulfide (or selenide, phosphide) precursors through coupling metal cations with anion groups of a functional polymer. Such an intimately binding carbon-metal-sulfide (or selenide, phosphide) interface structure enables the successful in situ generation of MC nanoparticles uniformly encapsulated into the carbon matrix just after a one-step carbonization treatment. The present synthetic strategy provides remarkable adjustability, predictability and generality to facilely fabricate a series of MC/C composites, offering sufficient freedom to explore their unique energy storage/conversation properties. As a proof of concept, the as-prepared SnS/C composite exhibits superior lithium ion and potassium ion storage capabilities when used as anode materials for alkali-metal ion batteries. The present work provides impressive insights into the design principles for MC/C composites that are the potential materials in targeted application fields, and opens up an efficacious avenue for their facile synthesis as well.

Ultrafast charge transfer at the electrode−electrolyte interface via an artificial dielectric layer

We report ultrafast charge transfer along the Li-ion pathway via a dielectric layer at the electrolyte−electrode interface. The rate capabilities of dielectric-modified cathodes were evaluated as a function of the combination of electrolyte solvents and dielectric layers having various dielectric constants, εr. The high-rate capability was enhanced when the loaded dielectric layer's εr was close to that of the electrolyte solvent. Density functional theory molecular dynamics calculations and atomic force microscope force measurements confirmed that the activation energies related to Li+ adsorption and desolvation were notably reduced when the loaded dielectric layer's εr approached that of the solvent. In this case, the solvated Li+ ion preferentially underwent desolvation on the solid surface. The discharge capacity at the ultrahigh rate of 50 C (1 C = 160 mA g−1) of the dielectric-modified thin films steadily increased with increasing length of the dielectrics−electrode−electrolyte triple-phase interface (TPI). Experimental and computational results reveal that, when the permittivity of the dielectrics and solvent are similar, solvated Li+ ions are preferentially involved in the physical adsorption and desolvation on the dielectric surface, followed by Li+ intercalation near the TPI.

Approaching high-performance pouch cell via fast thermal dissipation and polarization-assisted ion migration

To achieve higher holisticcapability for lithium batteries via the simplest possible way is one of the most urgent issues for their industrialization. Here, a low-cost TiN (1 wt%) with excellent electrical/thermal conductivity and high permittivity was mixed into the positive electrodes of LiNi0.5Co0.2Mn0.3O2(NCM523)‖graphite pouch cells, delivering dramatically improved lithium battery performance, particularly in terms of cyclic stability, energy density, and thermal safety. The NCM523/TiN‖graphite cell delivered a capacity retention of 91.8% at 3C rate after 500 cycles (cf., only 85.0% before adding TiN). Importantly, the energy density for the pouch cell with TiN added was boosted by 15% at 5C. It is found that a small amount of TiN (1 wt%) not only facilitates electron transfer but also promotes faster heat-dissipation in the electrode during the charge/discharge process, beneficial for achieving fast Li-ion diffusion kinetics, balanced CEI/SEI-film evolution, and excellent cycling stability especially at high rates. High localized dielectric polarization induced by TiN enhances the Li-ions kinetics around the triple-phase interface of NCM523, TiN and electrolyte. This very convenient approach is highly feasible for producing much better and safer LIBs with little extra cost involved, which is key to industrialization.

Mesoporous CeO2–C hybrid spheres as efficient support for platinum nanoparticles towards methanol electrocatalytic oxidation

The development of direct methanol fuel cells (DMFCs) is partially limited by the poor kinetics of methanol oxidation reaction (MOR) at the anode side. It was reported that the interaction between Pt and CeO2 enhances the electrocatalytic performance of Pt catalyst for MOR. In this work, a hybrid material (CeO2–C) composed of CeO2 and carbon was successfully prepared by a simple hydrothermal method followed by calcination in inert atmosphere. The hierarchically porous nanostructure and especially good electronic conductivity of CeO2–C make it an excellent support for Pt particles for application in electrocatalytic process. TEM investigation reveals that triple-phase interface of Pt, carbon and CeO2 forms in Pt/CeO2–C catalyst. Performance of the as-prepared catalyst for MOR was studied in alkaline medium. The Pt/CeO2–C catalyst shows superior catalytic performance for MOR compared with Pt/CeO2 and the physical mixture of Pt/CeO2 and acetylene black (Pt/CeO2+C). The significantly improved performance can be attributed to the synergetic effect between Pt particles and CeO2–C support, and the better conductivity of CeO2–C. This study provides a possible method to expand the application potential of CeO2 materials in MOR, and may also be used in other electrocatalytic process.

The effects of BaTiO3 nanodots density support on epitaxial LiCoO2 thin-film for high-speed rechargeability

LiCoO2 (LCO) is one of the most promising cathode materials for Li ion batteries (LIBs). However, LCO shows a rate-limiting step of Li+ migration between electrode and electrolyte interfaces, requiring LIBs to be charged under low-current conditions. For next generation batteries, it will be necessary to meet the demand for a shorter charging-time. We investigated a support method for the LCO surface to improve high C-rate performance, and revealed that the Li+ intercalation/de-intercalation reaction into/from LCO was accelerated by the introduction of a BaTiO3-LCO-electrolyte interface (triple-phase interface; TPI), due to the electric field concentration near the TPI. In this report, we investigate the dependence of high C-rate performance on the density of surface BaTiO3 nanodots using epitaxial LiCoO2 thin films created via pulsed laser deposition (PLD). As the number of nanodots increased, so did discharge capacity at 50C, becoming saturated at surface coverage over 22%. However, at 100C, the discharge capacity decreased at surface coverage over 40%. These results indicate that coalescence of nanodots reduces not only the TPI length but also the electrochemically active range at quite high C-rate. Therefore, we infer that optimal surface coverage should be varied depending on the C-rate.

The effect of relative permittivity of surface supporting materials for high-speed rechargeable LiCoO2 cathode film

We previously reported that the BaTiO3(BTO)-LiCoO2(LCO)-electrolyte triple-phase interface (TPI) could play an important role in C-rate enhancement, however, why this supporting material is effective for high performance remains unknown. We focus on the relative permittivity of supporting materials using finite element method calculations and experiments. Calculations revealed that the electric field near the TPI was reinforced as the relative permittivity of supporting materials increased. Experimentally, we prepared nanodots and micropads deposited on LCO thin film using BTO and CeO2 as supporting materials to evaluate electrochemical properties and SEI formation mechanisms, respectively. High C-rate performance was improved by the introduction of CeO2 and BTO nanodots; however, only the BTO nanodots deposited on LCO could work at 100C. In addition, SEI around the TPI was quite thin around CeO2 and BTO pads, although 10 and 300 nm SEI were observed at the non-pad area of CeO2 and BTO, respectively. This indicated that the Li+ motion between electrolyte and electrode could be accelerated depending on the relative permittivity of supporting materials. The low SEI around the TPI of both the CeO2 and BTO pads suggested that the main reactions of Li+ insertion/de-insertion into/from LCO might be dominant around the TPI area.

Droplet dynamics on slippery surfaces: small droplet, big impact

The dynamic interaction of droplets with slippery surfaces is essential to various industrial applications, such as anti-icing, water-repellency or water-harvesting, anti-bacterial, and phase change heat transfer. With recent progress in materials, manufacturing as well as learning from nature, the physics of droplet dynamics has been greatly enriched owing to the emergence of peculiar wetting states manifested on bio-inspired textured surfaces. This review is devoted to the discussion of the recent progress made in the authors’ understanding of the dynamic interaction of small droplets with bio-inspired surfaces. Particular attention is given to droplet impact on slippery surfaces, such as superhydrophobic surfaces characterised with air–solid–liquid triple-phase interface and slippery lubricant infused porous surfaces characterised with the liquid/liquid two-phase interface. Droplet spreading, retraction, contact time, elastodynamics as well as oblique impact are systematically reviewed. Finally, the authors offer their perspectives on this important and highly multidisciplinary research area.

Enhancement of Ultrahigh Rate Chargeability by Interfacial Nanodot BaTiO3 Treatment on LiCoO2 Cathode Thin Film Batteries.

Nanodot BaTiO3 supported LiCoO2 cathode thin films can dramatically improve high-rate chargeability and cyclability. The prepared BaTiO3 nanodot is <3 nm in height and 35 nm in diameter, and its coverage is <5%. Supported by high dielectric constant materials on the surface of cathode materials, Li ion (Li+) can intercalate through robust Li paths around the triple-phase interface consisting of the dielectric, cathode, and electrolyte. The current concentration around the triple-phase interface is observed by the finite element method and is in good agreement with the experimental data. The interfacial resistance between the cathode and electrolyte with nanodot BaTiO3 is smaller than that without nanodot BaTiO3. The decomposition of the organic solvent electrolyte can prevent the fabrication of a solidelectrolyte interface around the triple-phase interface. Li+ paths may be created at non solid electrolyte interface covered regions by the strong current concentration originating from high dielectric constant materials on the cathode. Robust Li+ paths lead to excellent chargeability and cyclability.

Freezing Site of Super-Cooled Water and Failure Mechanism of Cold Start of PEFC

The freezing site of super-cooled water and its effects on the triple-phase interface and pathway during PEFC cold start remain a subject of controversy. To address this deficiency, dynamic electrochemical characterizations, such as cyclic voltammetry (H2/N2), galvanostatic (H2/Air) and potentiostatic EIS (H2/N2, at OCV and 0.2 V), are attempted during the isothermal sub-zero operation of a fuel cell from the start till the failure. The EIS data are fitted using transmission line models, revealing a significant rise in mass transfer resistance and rather constant ohmic, activation resistance and proton conductivity of catalyst layer. Consecutive CVs before, during and after the cell failure show no change. These results show that ice does not cover the surface of the catalysts, but rather blocks the pores of cathode catalyst layer, resulting in the suffocation of oxygen supply to the reaction sites.

Enhanced synergistic catalysis by a novel triple-phase interface design of NiO/Ru@Ni for the hydrogen evolution reaction

High-efficiency synergistic catalysis was realized by a novel triple-phase interface design of the bifunctional catalysts of NiO and Ru nanoparticles, leading to simultaneous enhancement of all elementary steps.

(Invited) Preparation and Active Sites of Pyrolyzed Fe/N/C Non-Precious Metal Catalysts for Oxygen Reduction Reaction

Pyrolyzed Fe/N/C catalyst is a promising non-precious-metal catalyst (NPMCs) for oxygen reduction reaction (ORR) in fuel cells, and substantial progresses have been made recently in this field. However, to meet the requirement of practical applications, it still needs to improve significantly the ORR activity and stability of Fe/N/C catalyst, and to reveal active site structure for rational design of NPMCs. Herein, we report our recent progresses in preparation of high performance Fe/N/C catalyst and investigation of the active sites. The Fe/N/C catalyst was prepared by using precursor of poly-m-phenylenediamine (PmPDA), and exhibits an ORR performance of 12.8 A/g@0.80V in acid solution with H2O2 yield less than 1%.[1] When it was applied in proton exchange membrane cell (PEMFC), a peak power density could reach 0.81 W cm-2. We further improved the catalytic activity of PmPDA-Fe/N/C by co-doping of S element. The peak power density has exceed 1.0 W cm- 2. [2] The active sites of Fe/N/C catalyst are mostly located in micropores, so they are susceptible to water flooding. To this point, surface fluorination was applied to the Fe/N/C catalysts (such as the modification of Ar-CF3). The fluorinated Fe/N/C could perform stably over 120 h at 0.5 V with a current density of 0.56 A cm-2 in a H2-O2 PEMFC.[3] Alternatively, hydrophobic dimethyl silicon oil (DMS) was introduced into Fe/N/C catalyst layer, which leads to form triple-phase interface in micropores. As a result, Fe/N/C-based direct methanol fuel cell (DMFC) could yield a performance close to that of Pt-based DMFC.[4] We developed a molecule/ion probe method to study the active sites. It has revealed that the ORR activity of PmPDA-Fe/N/C is not sensitive to CO and NOx, but can be suppressed significantly by halide ions (e.g., Cl−, F−, and Br−) and low valence state sulfur-containing species (e.g., SCN−, SO2 , and H2S).[1] This indicates that the active sites of the Fe/N/C catalyst contains Fe element in acid medium. We further designed a single-atomic-layer Fe/N/C model catalyst based on monolayer graphene (FeNMLG), and systematically investigated the effects of defect density, the number of graphene layers, and the doped nitrogen species on the ORR activity.[5] It has demonstrated also that such Fe/N/C model catalyst is also fit for in situ vibration spectroscopic studies. Acknowledgement: This study was supported by grants from National Key Research and Development Program of China (2016YFB0101202), and NSFC (91645121 and 21621091).

Efficient removal of anti-inflammatory phenylbutazone from an aqueous solution employing a composite material based on poly(aniline-co-pyrrole)/multi-walled carbon nanotubes

Poly(Ani-co-Py)/MWCNT was synthesized by chemical oxidation in a triple-phase interface system and presented a high capacity for the removal of PBZ from wastewater.

Mesoporous 3D nitrogen-doped yolk-shelled carbon spheres for direct methanol fuel cells with polymer fiber membranes

In recent years, the oxygen reduction reaction (ORR) performance of N-doped carbon catalysts has made great progress and is now comparable to that of commercial Pt/C. However, the rich porous features of N-doped carbon catalysts give them a very low bulk density, which leads to problems in oxygen mass transfer due to water flooding when they are used as the cathode in fuel cells. Herein, a controllable mesoporous surface of nitrogen-doped yolk-shelled carbon spheres (N-YS-CS) with an extremely regular particle size was synthesized. The mesopores of the N-YS-CS effectively store the hydrophobic agent and form robust hydrophobic tunnels within the close-packed N-YS-CS units, forming an excellent structure for mass transfer at the cathode. This elaborately constructed hydrophobic cathode structure has a larger triple-phase interface than traditional carbon paper in an electrolyte-penetration-type direct methanol fuel cell (DMFC) with a polymer fiber membrane (PFM), and delivers a high power density (55 and 142 mW cm−2 at 25 °C and 60 °C, respectively) and discharge stability (a run time of over 210 h).

Constructing a Triple-Phase Interface in Micropores to Boost Performance of Fe/N/C Catalysts for Direct Methanol Fuel Cells

Pyrolyzed Fe/N/C, a promising nonprecious-metal catalyst for oxygen reduction reaction (ORR), usually relies on abundant micropores, which can host a large amount of active sites. However, microporous structure suffers from severe water flooding to break the triple-phase interface where ORR occurs, especially in a direct methanol fuel cell (DMFC) fed with liquid fuel. Current studies about the fabrication of a triple-phase interface are mainly limited on a Pt/C catalyst layer, where mesopores and macropores are concerned. Here, we successfully constructed a triple-phase interface in micropores of Fe/N/C catalysts by controlling the distribution of a hydrophobic additive, dimethyl silicon oil (DMS), just partially penetrating into the micropores. The elaborately constructed Fe/N/C-based DMFC can deliver high power density (102 and 130 mW cm–2 at 60 and 80 °C, respectively) and durability comparable to that of Pt/C-based DMFC. This study presents a new avenue to engineer catalyst microporous channels to boo...

Ionomer Layer Design of the Electrode Using PGM-Free Electrocatalysts for Anion Exchange Membrane Fuel Cells

An anion exchange membrane fuel cell (AEMFC) using hydrazine hydrate liquid fuel has been investigated1 and the concept vehicle was exhibited at the 42nd and 43rdTokyo Motor Show. For the improvement of the AEMFCs, technological breakthrough is necessary for anion exchange membranes. Especially, it is important to overcome the counteracting relationship of high ion conductance and low fuel permeability. In addition, technology to optimize triple-phase interface formed by catalyst and ionomer is important to improve the performance. Through many efforts to control the triple-phase interface, state-of-the-art proton exchange membrane fuel cell electrode that consists of platinum group electrocatalysts and perfluorinated ionomer has been investigated. However, the catalyst utilization is still insufficient. In addition, it is more challenging to control the triple phase interface for PGM-free electrocatalysts and anion exchange ionomers. Our approach to develop the advanced AEMFC is using anion conductive quaternized aromatic multiblock copolymers, poly(arylene ether)s (QPEs)2as shown in Fig.1 and controling the interface and PGM-free electrocatalysts both for anode and cathode. In this preliminary study, we focus on ionomer layer design for anode and cathode evaluated by ionomer thickness covered on the electrocatalysts. (TEM image shown in Fig.2) MEA consisting of the QPE membranes and ionomers was fabricated with PGM-free electrocatalysts and subjected to performance evaluation. The MEA performance of direct hydrazine fuel cells will be shown and discussed in the meeting. Figure captions Fig. 1. Representative structure of QPEs.3 Fig. 2. TEM Image of ionomer layer (red line) on the cathode catalysts.

Cathode Design of Non PGM Catalysts for Anion Exchange Membrane Fuel Cells

An anion exchange membrane fuel cell (AEMFC) using hydrazine hydrate liquid fuel has been developed1 and the concept vehicle powered by Precious Metal free Liquid Feed Fuel Cell (PMfLFC) was exhibited at the 42nd and 43rd Tokyo Motor Show. For the improvement of the AEMFCs, technological breakthrough is necessary for anion exchange membranes. Especially, it is important to overcome the counteracting relationship of high ion conductance and low fuel permeability. And also, technology to control triple-phase interface formed by catalyst and ionomer is important to improve the performance. Our approach to develop the advanced AEMFC is using anion conductive quaternized aromatic multiblock copolymers, poly(arylene ether)s (QPEs)2 and control the interface and non platinum group metal (PGM) catalysts both for anode and cathode. Even for state-of-the-art proton exchange membrane fuel cell electrode that consists of Pt catalysts and perfluorinated ionomer, triple-phase interface has not been optimized and the catalyst utilization is still insufficient. This is also the case for non PGM catalyst and anion exchange ionomer. In this paper, for the preliminary study we focus on electrode design for cathode and evaluated pore size of non PGM catalyst and distribution of catalyst in the ionomer solution. The non PGM catalysts formed meso pores (10-100nm) with the mixture of carbon materials. There are two different types of pores, one is inside of the catalyst itself and the other one is outside pores between the catalysts. Mixture of non PGM catalyst and carbon materials are increased meso pores. Oxygen reduction reaction (ORR) activity of the non-PGM catalysts was evaluated in 1M KOH aq using rotating ring disk electrode (RRDE). The Fe-N-C non-PGM catalyst showed comparable ORR performance to platinum/carbon catalyst (TEV10V60E). The half wave potentials were 0.886V and 0.849V, respectively. MEA consists of the QPE membranes and ionomers were fabricated with non-PGM catalysts and subjected to performance evaluation. The MEA performance of direct hydrazine fuel cells will be shown and discussed in the meeting.