Membraneless vanadium redox fuel cell using laminar flow

Polymeric Microfluidic Fuel Cells with Controlled Printed Patterns

Since its first appearance almost a couple of decades ago, microfluidic fuel cells (MFFCs) have gained considerable research momentum due to their potential applications in portable devices. The main focus has been on the effective fabrication of microfluidic channels with different materials, where the manufacturing limitations proved to be the main stumbling blocks. Paper-based MFFCs have been reported with some success, where the porosity of the flow channel medium drives the reactants, greatly reducing the need for elaborate external devices and complex manufacturing obstacles, although the longevity of these cells remains questionable. The current article addresses this issue by replacing the paper-based flow channels with 3D-printed substrates of different structural forms to serve as pathways for controlled flow and mixing responses of the reactant liquids without the use of other devices, such as micro pumps and valves. The line-by-line material consolidation mechanics of fused filament fabrication and the porous mesostructural responses of a commercial polymer filament are combined to build the microfluidic fuel channels of varying configurations. Numerical and experimental characterizations proved the cells to perform better than the current paper-based counterparts, apart from better longevity and possible new opportunities for future improvements based on more complex micro-, meso-, and macrostructural advances.

Physics, electrochemistry, chemistry, and electronics of the vanadium redox flow battery by analyzing all the governing equations

The vanadium redox flow battery has been intensively examined since the 1970s, with researchers looking at its electrochemical time varying electrolyte concentration time variation equations (both tank and cells, for negative and positive half cells), its thermal time variation equations, and fluid flow equations. Chemistry behavior of the electrolyte ions have also been intensively examined too. In this perspective, all of the phenomena have been examined, unified and presented together with their physical chemistry shown in the appropriate equations. This is done by providing the field equations for the battery, which are electronic, electrochemical, chemical, physics of fluid dynamics, and thermal physics of heat transport, in character. They are interpreted in new analytical equations providing fundamental scientific insights, as well as allowing engineering and manufacturing assessments. Graphs of pressure trend in electrodes, power and temperature tables for electrode and current collector loss mechanisms are provided. Current density, concentration, electrostatic, and overpotential functional dependences in the electrodes are given.

Optimization of Multiple Reactants in a Membrane-Less Direct Methanol Fuel Cell (DMFC)

Membrane-less fuel cells are a promising power source for portable applications that enable the solving of membrane-related issues, such as water management and high cost, in conventional fuel cells. Apparently, research on this system uses a single electrolyte. This study focused on enhancing the performance of membrane-less fuel cells by introducing multiple reactants that are dual electrolytes with hydrogen peroxide (H₂O₂) and oxygen as oxidants in membrane-less direct methanol fuel cells (DMFC). The conditions tested for the system are (a) acidic, (b) alkaline, (c) dual medium with oxygen as an oxidant, and (d) dual medium and dual oxygen and hydrogen peroxide as an oxidant. Additionally, the effect of fuel utilization on different electrolyte and fuel concentrations was also studied. It was found that the fuel utilization decreases dramatically with the increasing of the fuel concentration, but it improved with the increasing of the electrolyte concentration until 2M. The performance of the dual oxidants in dual-electrolyte membrane-less DMFCs was 15.5 mW cm^-2 of the power density achieved before optimization. Later, the system was optimized, and the power density increased to 30 mW cm^-2. Finally, this work presented the stability of the cell using the suggested parameters from the optimization process. This study indicated that the performance of the membrane-less DMFC increased for dual electrolytes with mixed oxygen and hydrogen peroxide as oxidants compared to a single electrolyte.

Developing a novel computer numerical control-fabricated laminar-flow microfluidic microbial fuel cells as the bioelectrochemical sensor and power source: Enrichment, operation, and Cr(VI) detection

By introducing the computer numerical control (CNC) engraving technology, this study fabricated the reusable CNC-fabricated membrane-less laminar flow microfluidic MFC (LMMFC) to develop the bioelectrochemical sensor and power source simultaneously. To verify its applicability, optimization of electroactive bacteria (EAB) cultivation and laminar-flow formation, performance of power density and long-term operation, and detection of Cr(VI) were evaluated. Results of EAB optimization showed under lower external resistance, shorter start-up time of current production, larger oxidation current, denser microbial distribution, and a higher percentage of Geobacter spp. were observed. Results of the laminar-flow operation indicated that increasing the density difference between two solutions and raising the anode flow velocity can minimize the interference of the diffusion zone. The power output of LMMFC could reach 2085 mW m^-2 and achieve long-term stability for current production (∼150 h). Regarding the detection of Cr(VI), low-concentration (0.1∼1 ppm) and high-concentration (1-10 ppm) ranges reached the linear coefficient of determination of 0.98 and 0.97, respectively. Overall, these results suggest that an LMMFC which can both act as the power source and biosensor was successfully developed, showing potential for future self-power application.

Fuel Cell Reactors for the Clean Cogeneration of Electrical Energy and Value-Added Chemicals

Fuel cell reactors can be tailored to simultaneously cogenerate value-added chemicals and electrical energy while releasing negligible CO2 emissions or other pollution; moreover, some of these reactors can even “breathe in” poisonous gas as feedstock. Such clean cogeneration favorably offsets the fast depletion of fossil fuel resources and eases growing environmental concerns. These unique reactors inherit advantages from fuel cells: a high energy conversion efficiency and high selectivity. Compared with similar energy conversion devices with sandwich structures, fuel cell reactors have successfully “hit three birds with one stone” by generating power, producing chemicals, and maintaining eco-friendliness. In this review, we provide a systematic summary on the state of the art regarding fuel cell reactors and key components, as well as the typical cogeneration reactions accomplished in these reactors. Most strategies fall short in reaching a win–win situation that meets production demand while concurrently addressing environmental issues. The use of fuel cells (FCs) as reactors to simultaneously produce value-added chemicals and electrical power without environmental pollution has emerged as a promising direction. The FC reactor has been well recognized due to its “one stone hitting three birds” merit, namely, efficient chemical production, electrical power generation, and environmental friendliness. Fuel cell reactors for cogeneration provide multidisciplinary perspectives on clean chemical production, effective energy utilization, and even pollutant treatment, with far-reaching implications for the wider scientific community and society. The scope of this review focuses on unique reactors that can convert low-value reactants and/or industrial wastes to value-added chemicals while simultaneously cogenerating electrical power in an environmentally friendly manner.

Graphical Abstract

A schematic diagram for the concept of fuel cell reactors for cogeneration of electrical energy and value-added chemicals

High Power Density Direct Formate Microfluidic Fuel Cells with the Different Catalyst-Free Oxidants

As micropower devices, microfluidic fuel cells (MFCs) have gained much attention due to their simple configurations and high power densities. MFCs exploit the parallel laminar flowing of two electrolytes in a microchannel with a characteristic length from 1 to 1000 μm to separate the anolyte and catholyte, without the proton exchange membranes in the traditional fuel cells. These membrane-less configurations can avoid a series of technical problems related to the membranes. To achieve an MFC with high power density and low cost, we constructed the direct formate MFCs with two catalyst-free oxidants containing FeCl₃ and Na₂S₂O₈ solutions, respectively, and compared the performance of the two MFCs. Due to Na₂S₂O₈ being an oxidant with some distinctive advantages, including its high theoretical potential, high solubility of itself and its reduction product, and environmental friendliness, the Na₂S₂O₈-based MFC showed a higher open-circuit voltage (>2.0 V) and better performance. Then, we studied the effects of oxidant concentrations, flow rates, and fuel concentrations on the performance of the Na₂S₂O₈-based MFC. The results showed the optimum performance of the Na₂S₂O₈-based MFC with the peak power density of 214.95 mW cm^-2 and the limiting current density of 700.13 mA cm^-2 under the conditions of 1.5 M HCOONa, 2 M Na₂S₂O₈, and 300 μL min^-1 at an anolyte/catholyte flow ratio of 2:1. The performance was also the highest among the direct formate MFCs reported up to now. Moreover, the Na₂S₂O₈-based MFC could stably discharge for about 4 h under a constant voltage. All of the results demonstrated that Na₂S₂O₈ was a suitable oxidant and that the Na₂S₂O₈-based MFC could realize the goals of high power density and low cost for the actual application of MFCs.

Development and Challenges of Biphasic Membrane-Less Redox Batteries

Ion exchange membranes (IEMs) play important roles in energy generation and storage field, such as fuel cell, flow battery, however, a major barrier in the way of large-scale application is the high cost of membranes (e.g., Nafion membranes price generally exceeds USD$ 200 m-2 ). The membrane-less technology is one of the promising approaches to solve the problem and thus has attracted much attention and been explored in a variety of research paths. This review introduces one of the representative membrane-less battery types, Biphasic membrane-less redox batteries that eliminate the IEMs according to the principle of solvent immiscibility and realizes the phase splitting in a thermodynamically stable state. It is systematically classified and summarizes their performances as well as the problems they are suffering from, and then several effective solutions are proposed based on the modification of electrodes and electrolytes. Finally, special attention is given to the challenges and prospects of Biphasic membrane-less redox batteries, which could contribute to the development of membrane-less batteries.

In operando visualization of redox flow battery in membrane-free microfluidic platform

The current study investigates fundamentals of electrochemical reactions using the membrane-free redox flow battery (RFB) platform with a laminar strategy and colorimetry of multiredox organic molecules. Taking advantage of unique color changes of electrolytes depending on the state of charge, we analyze the electrochemical kinetics of the RFB system in terms of charge and mass transfer. It is verified that a balanced rate of charge and mass transfer significantly affects the battery performance. Furthermore, a classical physicochemical hydrodynamic equation is adopted for scaling analysis of the depletion region deteriorating battery performance. We successfully integrate analytical, numerical, and experimental data for elucidating the depletion region. Based on these fundamental studies, finally, a favorable design is suggested for performance enhancement.

Redox flow batteries (RFBs) are attractive large-scale energy storage techniques, achieving remarkable progress in performance enhancement for the last decades. Nevertheless, an in-depth understanding of the reaction mechanism still remains challenging due to its unique operation mechanism, where electrochemistry and hydrodynamics simultaneously govern battery performance. Thus, to elucidate the precise reactions occurring in RFB systems, an appropriate analysis technique that enables the real-time observation of electrokinetic phenomena is indispensable. Herein, we report in operando visualization and analytical study of RFBs by employing a membrane-free microfluidic platform, that is, a membrane-free microfluidic RFB. Using this platform, the electrokinetic investigations were carried out for the 5,10-bis(2-methoxyethyl)-5,10-dihydrophenazine (BMEPZ) catholyte, which has been recently proposed as a high-performance multiredox organic molecule. Taking advantage of the inherent colorimetric property of BMEPZ, we unravel the intrinsic electrochemical properties in terms of charge and mass transfer kinetics during the multiredox reaction through in operando visualization, which enables theoretical study of physicochemical hydrodynamics in electrochemical systems. Based on insights on the electrokinetic limitations in RFBs, we verify the validity of electrode geometry design that can suppress the range of the depletion region, leading to enhanced cell performance.

Deep learning-based optimization of a microfluidic membraneless fuel cell for maximum power density via data-driven three-dimensional multiphysics simulation

A deep learning-based method for optimizing a membraneless microfluidic fuel cell (MMFC)performance by combining the artificial neural network (ANN) and genetic algorithm (GA) was for the first time introduced. A three-dimensional multiphysics model that had an accuracy equivalent to experimental results (R² = 0.976) was employed to generate the ANN's training data. The constructed ANN is equivalent to the simulation (R² = 0.999) but with far better computation resource efficiency as the ANN's execution time is only 0.041 s. The ANN model is then used by the GA to determine the inputs (microchannel length = 10.040 mm, width = 0.501 mm, height = 0.635 mm; temperature = 288.210 K, cell voltage = 0.309 V) that lead to the maximum power density of 0.263 mWcm^-2 (current density of 0.852 mAcm^-2) of the MMFC. The ANN-GA and numerically calculated maximum power densities differed only by 0.766%.

Optimization of a Membraneless Microfluidic Fuel Cell with a Double-Bridge Flow Channel

In this work, a design optimization study was conducted to improve the performance of a membraneless microfluidic fuel cell with a double-bridge cross-section of the flow channel. Governing equations including Navier–Stokes, mass-transport, and Butler–Volmer equations were solved numerically to analyze the electrochemical phenomena and evaluate the performance of the fuel cells. Optimization was performed to maximize the peak power density using a genetic algorithm combined with a surrogate model constructed by radial basis neural network. Two sub-channel widths of the flow channel were selected as design variables for the optimization. As a result, a large increase in the inner channel width and a small decrease in the outer channel width effectively increased the peak power density of the MMFC. The optimal design increased the peak power density by 57.6% compared to the reference design.

Microfluidics for Electrochemical Energy Conversion

Electrochemical energy conversion is an important supplement for storage and on-demand use of renewable energy. In this regard, microfluidics offers prospects to raise the efficiency and rate of electrochemical energy conversion through enhanced mass transport, flexible cell design, and ability to eliminate the physical ion-exchange membrane, an essential yet costly element in conventional electrochemical cells. Since the 2002 invention of the microfluidic fuel cell, the research field of microfluidics for electrochemical energy conversion has expanded into a great variety of cell designs, fabrication techniques, and device functions with a wide range of utility and applications. The present review aims to comprehensively synthesize the best practices in this field over the past 20 years. The underlying fundamentals and research methods are first summarized, followed by a complete assessment of all research contributions wherein microfluidics was proactively utilized to facilitate energy conversion in conjunction with electrochemical cells, such as fuel cells, flow batteries, electrolysis cells, hybrid cells, and photoelectrochemical cells. Moreover, emerging technologies and analytical tools enabled by microfluidics are also discussed. Lastly, opportunities for future research directions and technology advances are proposed.

Fused Filament Fabrication (FFF) for Manufacturing of Microfluidic Micromixers: An Experimental Study on the Effect of Process Variables in Printed Microfluidic Micromixers

The need for accessible and inexpensive microfluidic devices requires new manufacturing methods and materials as a replacement for traditional soft lithography and polydimethylsiloxane (PDMS). Recently, with the advent of modern additive manufacturing (AM) techniques, 3D printing has attracted attention for its use in the fabrication of microfluidic devices and due to its automated, assembly-free 3D fabrication, rapidly decreasing cost, and fast-improving resolution and throughput. Here, fused filament fabrication (FFF) 3D printing was used to create microfluidic micromixers and enhance the mixing process, which has been identified as a challenge in microfluidic devices. A design of experiment (DoE) was performed on the effects of studied parameters in devices that were printed by FFF. The results of the colorimetric approach showed the effects of different parameters on the mixing process and on the enhancement of the mixing performance in printed devices. The presence of the geometrical features on the microchannels can act as ridges due to the nature of the FFF process. In comparison to passive and active methods, no complexity was added in the fabrication process, and the ridges are an inherent property of the FFF process.

Flow Configurations of Membraneless Microfluidic Fuel Cells: A Review

Membraneless microfluidic fuel cells (MMFCs) are being studied extensively as an alternative to batteries and conventional membrane fuel cells because of their simple functioning and lower manufacturing cost. MMFCs use the laminar flow of reactant species (fuel and oxidant) to eliminate the electrolyte membrane, which has conventionally been used to isolate anodic and cathodic half-cell reactions. This review article summarizes the MMFCs with six major categories of flow configurations that have been reported from 2002 to 2020. The discussion highlights the critical factors that affect and limit the performance of MMFCs. Since MMFCs are diffusion-limited, most of this review focuses on how different flow configurations act to reduce or modify diffusive mixing and depletion zones to enhance the power density output. Research opportunities are also pointed out, and the challenges in MMFCs are suggested to improve cell performance and make them practical in the near future.

A Single-Flow Battery with Multiphase Flow

Widespread adoption of redox flow batteries (RFBs) for renewable energy storage is inhibited by a relatively high cost of storage. This is due largely to typical RFBs requiring two flows, two external tanks, and expensive ion-exchange membranes. Here, we propose a potentially inexpensive Zn-Br₂ RFB which is membraneless and requires only a single flow. The flow is an emulsion consisting of a continuous, Br₂ -poor aqueous phase and a dispersed, Br₂ -rich polybromide phase, pumped through the channel separating anode and cathode. With our prototype cell, we explore the effect of polybromide-phase volume fraction and Br₂ concentration on cell performance and plating efficiencies. We demonstrate high discharge currents of up to 270 mA/cm² , plating efficiencies up to 88 %, and dendriteless plating up to the highest Zn loading investigated of 250 mAh/cm² . We provide mechanistic insights into cell behavior and elucidate paths towards unlocking ultra-low-cost single-flow RFBs with multiphase flow.

Recent Advances in Electrochemical Monitoring of Chromium

The extensive use of chromium by several industries conducts to the discharge of an immense quantity of its various forms in the environment which affects drastically the ecological and biological lives especially in the case of hexavalent chromium. Electrochemical sensors and biosensors are useful devices for chromium determination. In the last five years, several sensors based on the modification of electrode surface by different nanomaterials (fluorine tin oxide, titanium dioxide, carbon nanomaterials, metallic nanoparticles and nanocomposite) and biosensors with different biorecognition elements (microbial fuel cell, bacteria, enzyme, DNA) were employed for chromium monitoring. Herein, recent advances related to the use of electrochemical approaches for measurement of trivalent and hexavalent chromium from 2015 to 2020 are reported. A discussion of both chromium species detections and speciation studies is provided.

Recent Advances in the Development of Organic and Organometallic Redox Shuttles for Lithium-Ion Redox Flow Batteries

In recent years, redox flow batteries (RFBs) and derivatives have attracted wide attention from academia to the industrial world because of their ability to accelerate large-grid energy storage. Although vanadium-based RFBs are commercially available, they possess a low energy and power density, which might limit their use on an industrial scale. Therefore, there is scope to improve the performance of RFBs, and this is still an open field for research and development. Herein, a combination between a conventional Li-ion battery and a redox flow battery results in a significant improvement in terms of energy and power density alongside better safety and lower cost. Currently, Li-ion redox flow batteries are becoming a well-established subdomain in the field of flow batteries. Accordingly, the design of novel redox mediators with controllable physical chemical characteristics is crucial for the application of this technology to industrial applications. This Review summarizes the recent works devoted to the development of novel redox mediators in Li-ion redox flow batteries.

General Growth of Carbon Nanotubes for Cerium Redox Reactions in High-Efficiency Redox Flow Batteries

Carbon nanotubes (CNTs) possess remarkable mechanical, electrical, thermal, and optical properties that predestine them for numerous potential applications. The conventional chemical vapor deposition (CVD) route for the production of CNTs, however, suffers from costly and complex issues. Herein, we demonstrate a general and high-yield strategy to grow nitrogen-doped CNTs (NCNTs) on three-dimensional (3D) graphite felt (GF) substrates, through a direct thermal pyrolysis process simply using a common tube furnace, instead of the costly and complex CVD method. Specifically, the NCNTs-decorated GF (NCNT-GF) electrode possesses enhanced electrocatalytic performance towards cerium redox reactions, mainly due to the catalytic effect of N atoms doped into NCNTs, and ingenious and hierarchical 3D architecture of the NCNT-GF. As a result, the cell with the NCNT-GF serving as a positive electrode shows the improved energy efficiency with increases of about 53.4% and 43.8% over the pristine GF and the acidly treated GF at a high charge/discharge rate of 30 mA cm⁻², respectively. Moreover, the as-prepared NCNT catalyst-enhanced electrode is found to be highly robust and should enable a long-term cycle without detectable efficiency loss after 500 cycles. The viable synthetic strategy reported in this study will contribute to the further development of more active heteroatom-doped CNTs for redox flow batteries.

Paper-Based Microfluidics for Electrochemical Applications

Paper-based microfluidics is characteristic of fluid transportation through spontaneous capillary action of paper and has exhibited great promise for a variety of applications especially for sensing. Furthermore, paper-based microfluidics enables the design of miniaturized electrochemical devices to be applied in the energy sector, which is especially attractive for the rapid growing market of small size disposable electronics. This review gives a brief summary on the basics of paper chemistry and capillary-driven microfluidic behavior, and highlights recent advances of paper-based microfluidics in developing electrochemical sensing devices and miniaturized energy storage/conversion devices. Their structural features, working principles and exemplary applications are comprehensively elaborated and discussed. Additionally, this review also points out the existing challenges and future opportunities of paper-based microfluidic electronics.

Effects of Bridge-Shaped Microchannel Geometry on the Performance of a Micro Laminar Flow Fuel Cell

A laminar flow micro fuel cell comprising of bridge-shaped microchannel is investigated to find out the effects of the cross-section shape of the microchannel on the performance. A parametric study is performed by varying the heights and widths of the channel and bridge shape. Nine different microchannel cross-section shapes are evaluated to find effective microchannel cross-sections by combining three bridge shapes with three channel shapes. A three-dimensional fully coupled numerical model is used to calculate the fuel cell's performance. Navier-Stokes, convection and diffusion, and Butler-Volmer equations are implemented using the numerical model. A narrow channel with a wide bridge shape shows the best performance among the tested nine cross-sectional shapes, which is increased by about 78% compared to the square channel with the square bridge shape.

Enhanced Oxygen Solubility in Metastable Water under Tension

Despite its relevance in numerous natural and industrial processes, the solubility of molecular oxygen has never been directly measured in capillary-condensed liquid water. In this article, we measure oxygen solubility in liquid water trapped within nanoporous samples, in metastable equilibrium with a subsaturated vapor. We show that solubility increases two fold at moderate subsaturations (relative humidity ∼0.55). This evolution with relative humidity is in good agreement with a simple thermodynamic prediction using properties of bulk water, previously verified experimentally at positive pressure. Our measurement thus verifies the validity of this macroscopic thermodynamic theory to strong confinement and large negative pressures, where significant nonidealities are expected. This effect has strong implications for important oxygen-dependent chemistries in natural and technological contexts.

Advances in electro- and sono-microreactors for chemical synthesis

The anatomy of electrochemical flow microreactors is important to safely perform chemical reactions in order to obtain pure and high yielding substances in a controlled and precise way that excludes the use of supporting electrolytes. Flow microreactors are advantageous in handling unstable intermediates compared to batch techniques and have efficient heat/mass transfer. Electrode nature (cathode and anode) and their available exposed surface area to the reaction mixture, parameters of the spacer, flow rate and direction greatly affects the efficiency of the electrochemical reactor. Solid formation during reactions may result in a blockage and consequently decrease the overall yield, thus limiting the use of microreactors in the field of electrosynthesis. This problem could certainly be overcome by application of ultrasound to break the solids for consistent flow. In this review, we discuss in detail the aforementioned issues, the advances in microreactor technology for chemical synthesis, with possible application of sonochemistry to deal with solid formations. Various examples of flow methods for electrosynthesis through microreactors have been explained in this review, which would definitely help to meet future demands for efficient synthesis and production of various pharmaceuticals and fine chemicals.

Label-Free Monitoring of Diffusion in Microfluidics

Label-free, real-time detection of concentration gradients is demonstrated in a microfluidic H-filter, using an integrated photonic crystal slab sensor to monitor sample refractive index with spatial resolution. The recorded diffusion profiles reveal root-mean-square diffusion lengths for non-fluorescing and non-absorbing molecules, both small (glucose, 180 Da) and large (bovine serum albumin, 67 kDa).

Microtubular Fuel Cell with Ultrahigh Power Output per Footprint

A novel realization of microtubular direct methanol fuel cells (µDMFC) with ultrahigh power output is reported by using "rolled-up" nanotechnology. The microtube (Pt-RuO₂ -RUMT) is prepared by rolling up Ru₂ O layers coated with magnetron-sputtered Pt nanoparticles (cat-NPs). The µDMFC is fabricated by embedding the tube in a fluidic cell. The footprint of per tube is as small as 1.5 × 10^-4 cm² . A power density of ≈257 mW cm^-2 is obtained, which is three orders of magnitude higher than the present microsized DFMCs. Atomic layer deposition technique is applied to alleviate the methanol crossover as well as improve stability of the tube, sustaining electrolyte flow for days. A laminar flow driven mechanism is proposed, and the kinetics of the fuel oxidation depends on a linear-diffusion-controlled process. The electrocatalytic performance on anode and cathode is studied by scanning both sides of the tube wall as an ex situ working electrode, respectively. This prototype µDFMC is extremely interesting for integration with micro- and nanoelectronics systems.

A Membrane-Free Redox Flow Battery with Two Immiscible Redox Electrolytes

Flexible and scalable energy storage solutions are necessary for mitigating fluctuations of renewable energy sources. The main advantage of redox flow batteries is their ability to decouple power and energy. However, they present some limitations including poor performance, short‐lifetimes, and expensive ion‐selective membranes as well as high price, toxicity, and scarcity of vanadium compounds. We report a membrane‐free battery that relies on the immiscibility of redox electrolytes and where vanadium is replaced by organic molecules. We show that the biphasic system formed by one acidic solution and one ionic liquid, both containing quinoyl species, behaves as a reversible battery without any membrane. This proof‐of‐concept of a membrane‐free battery has an open circuit voltage of 1.4 V with a high theoretical energy density of 22.5 Wh L⁻¹, and is able to deliver 90 % of its theoretical capacity while showing excellent long‐term performance (coulombic efficiency of 100 % and energy efficiency of 70 %).

Platinum-based heterogeneous nanomaterials via wet-chemistry approaches toward electrocatalytic applications

The heterogeneously structured nanomaterials usually exhibit enhanced catalytic properties in comparison with each one of the constituent materials due to the synergistic effect among their different domains. Within the last decade, the development of wet-chemistry methods leads to the blossom of research in materials with heterogeneous nanostructures, which creates great opportunities also a tremendous challenge to apply these materials for highly efficient energy conversion. We herein would systematically introduce the latest research developments in Pt-based nanomaterials with heterogeneous structures, e.g. core-shell, hollow interiors, stellated/dendritic morphologies, dimeric, or composite construction, and their potential applications as electrocatalysts toward direct methanol fuel cell reactions, including methanol oxidation reaction and oxygen reduction reaction in acidic conditions, aiming at the summarization of the fundamentals and technical approaches in synthesis, fabrication and processing of heterogeneous nanomaterials so as to provide the readers a systematic and coherent picture of the filed. This review will focus on the intrinsic relationship between the catalytic properties and the physical or/and chemical effects in the heterogeneous nanomaterials, providing for technical bases for effectively developing novel electrocatalyts with low cost, enhanced activity and high selectivity.