Non-Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes

A versatile optimization framework for porous electrode design

Porous electrodes are performance-defining components in electrochemical devices, such as redox flow batteries, as they govern the electrochemical performance and pumping demands of the reactor. Yet, conventional porous electrodes used in redox flow batteries are not tailored to sustain convection-enhanced electrochemical reactions. Thus, there is a need for electrode optimization to enhance the system performance. In this work, we present an optimization framework to carry out the bottom-up design of porous electrodes by coupling a genetic algorithm with a pore network modeling framework. We introduce geometrical versatility by adding a pore merging and splitting function, study the impact of various optimization parameters, geometrical definitions, and objective functions, and incorporate conventional electrode and flow field designs. Moreover, we show the need for optimizing geometries for specific reactor architectures and operating conditions to design next-generation electrodes, by analyzing the genetic algorithm optimization for initial starting geometries with diverse morphologies (cubic and a tomography-extracted commercial electrode), flow field designs (flow-through and interdigitated), and redox chemistries (VO2+/VO2 + and TEMPO/TEMPO+). We found that for kinetically sluggish electrolytes with high ionic conductivity, electrodes with numerous small pores and high internal surface area provide enhanced performance, whereas for kinetically facile electrolytes with low ionic conductivity, low through-plane tortuosity and high hydraulic conductance are desired. The computational tool developed in this work can further expanded to the design of high-performance electrode materials for a broad range of operating conditions, electrolyte chemistries, reactor designs, and electrochemical technologies.

Carbon Structure Regulation Strategy for the Electrode of Vanadium Redox Flow Battery

Vanadium redox flow battery (VRFB) is a type of energy storage device known for its large-scale capacity, long-term durability, and high-level safety. It serves as an effective solution to address the instability and intermittency of renewable energy sources. Carbon-based materials are widely used as VRFB electrodes due to cost-effectiveness and well-stability. However, pristine electrodes need proper modification to overcome original poor hydrophilicity and fewer reaction active sites. Adjusting the carbon structure is recognized as a viable method to boost the electrochemical activity of electrodes. This review delves into the advancements in research related to ordered and disordered carbon structure electrodes including the adjusting methods, structural characteristics, and catalytic properties. Ordered carbon structures are categorized into nanoscale and macroscale orderliness based on size, leading to improved conductivity and overall performance of the electrode. Disordered carbon structures encompass methods such as doping atoms, grafting functional groups, and creating engineered holes to enhance active sites and hydrophilicity. Based on the current research findings on carbon electrode structures, this work puts forth some promising prospects for future feasibility.

A Thermal-Ball-Valve Structure Separator for Highly Safe Lithium-Ion Batteries

The separator located between the positive and negative electrodes not only provides a lithium-ion transmission channel but also prevents short circuits for direct contact of electrodes. The inferior dimension thermostability of commercial polyolefin separators intensifies the thermal runaway of batteries under abuse such as short circuits, overcharge, and so on. a polyvinylidene fluoride/polyether imide (PVDF/PEI) separator with high thermal stability in which the high thermostable PEI microspheres are evenly dispersed in the PVDF film matrix and also located in the micro holes of the PVDF film is developed. They not only function as strong skeleton that enables the rare shrink of the separator at 200 °C avoiding short circuit but also act as ball valve that blocks the lithium ion transmission channel at 150 °C interrupting the further heat aggregation. Thus, the LiNi0.6Co0.2Mn0.2O2/Li batteries exhibit high cycle stability of 96.5% capacity retention after 100 cycles at 0.2C and 80°C. Further, the LiNi0.6Co0.2Mn0.2O2/graphite pouch cells are constructed and deliver good safety performance without smoke release and catching fire after the nail penetration test.

Regulating microstructures of aerogels by controlling phase separation mechanism for improving specific surface area and energy harvesting

Aerogels with 3D porous structures have been attracting increasing attention among functional materials due to their advantages of being lightweight and high specific surface area. Precise control of the porous structure of aerogel is essential to improve its performance. In this work, polylactic acid (PLA) aerogels with distinctly different microstructures were fabricated by precisely controlling the phase separation behavior of the ternary solution system. Rheological and theoretical analyses have revealed that the interactions between polymer molecules, solvents and non-solvents play a crucial role in determining the nucleation and growth of poor olymer and rich polymer phases. By adjusting the non-solvent type and the solution composition, aerogels with spider network structure, bead-like connected microsphere structure, and cluster petal structure were obtained. Ideal spinodal phase separation conditions were obtained to produce aerogels with a homogeneous fiber network structure. The optimum PLA aerogel achieved an extremely porosity of 96 % and a high specific surface area of 114 m2/g, which rendered it with excellent triboelectric generation performance. Thus, this work provides fundamental insights into the precise regulation of the phase separation behavior and the structure of the aerogel, which can help boost the performance and expand the applications of PLA aerogels.

Engineering Lung-Inspired Flow Field Geometries for Electrochemical Flow Cells with Stereolithography 3D Printing

Electrochemical flow reactors are increasingly relevant platforms in emerging sustainable energy conversion and storage technologies. As a prominent example, redox flow batteries, a well-suited technology for large energy storage if the costs can be significantly reduced, leverage electrochemical reactors as power converting units. Within the reactor, the flow field geometry determines the electrolyte pumping power required, mass transport rates, and overall cell performance. However, current designs are inspired by fuel cell technologies but have not been engineered for redox flow battery applications, where liquid-phase electrochemistry is sustained. Here, we leverage stereolithography 3D printing to manufacture lung-inspired flow field geometries and compare their performance to conventional flow field designs. A versatile two-step process based on stereolithography 3D printing followed by a coating procedure to form a conductive structure is developed to manufacture lung-inspired flow field geometries. We employ a suite of fluid dynamics, electrochemical diagnostics, and finite element simulations to correlate the flow field geometry with performance in symmetric flow cells. We find that the lung-inspired structural pattern homogenizes the reactant distribution throughout the porous electrode and improves the electrolyte accessibility to the electrode reaction area. In addition, the results reveal that these novel flow field geometries can outperform conventional interdigitated flow field designs, as these patterns exhibit a more favorable balance of electrical and pumping power, achieving superior current densities at lower pressure loss. Although at its nascent stage, additive manufacturing offers a versatile design space for manufacturing engineered flow field geometries for advanced flow reactors in emerging electrochemical energy storage technologies.

Bayesian-Optimization-Assisted Laser Reduction of Poly(acrylonitrile) for Electrochemical Applications

Laser reduction of polymers has recently been explored to rapidly and inexpensively synthesize high-quality graphitic and carbonaceous materials. However, in past work, laser-induced graphene has been restricted to semiaromatic polymers and graphene oxide; in particular, poly(acrylonitrile) (PAN) is claimed to be a polymer that cannot be laser-reduced successfully to form electrochemically active material. In this work, three strategies to surmount this barrier are employed: (1) thermal stabilization of PAN to increase its sp² content for improved laser processability, (2) prelaser treatment microstructuring to reduce the effects of thermal stresses, and (3) Bayesian optimization to search the parameter space of laser processing to improve performance and discover morphologies. Based on these approaches, we successfully synthesize laser-reduced PAN with a low sheet resistance (6.5 Ω sq^-1) in a single lasing step. The resulting materials are tested electrochemically, and their applicability as membrane electrodes for vanadium redox flow batteries is demonstrated. This work demonstrates electrodes that are processed in air, below 300 °C, which are cycled stably over 2 weeks at 40 mA cm^-2, motivating further development of laser reduction of porous polymers for membrane electrode applications such as RFBs.

Architected Low-Tortuosity Electrodes with Tunable Porosity from Nonequilibrium Soft-Matter Processing

Mass transport is performance-defining across energy storage devices, often causing limitations at high current rates. To optimize and balance the device-scale energy and power density for a given energy storage demand, tailored electrode architectures with precisely controllable phase dimensions are needed in combination with low-tortuosity channels that maximize the geometric component of diffusion and species flux. A material-agnostic nonequilibrium soft-matter process is reported to fabricate free-standing inorganic composite electrodes with adjustable thicknesses of 100s of µm, featuring straight and accessible channels ranging in diameter from 5-30 µm, coupled with tunable material-to-pore ratios. Such architected anode and cathode electrodes exhibit electrochemical and architectural stability over extended cycling in a full-cell battery. Further, mass-transport constraints appear at high current densities, and the lithiation step is identified as rate-performance limiting, a result of insufficient lithium-ion supply and concentration polarization. The results demonstrate the need for and feasibility of tailored electrode architectures where dimensional ratios between low-tortuosity channels, the charge-storing matrix, and electrode thickness are tunable to meet coupled power and energy-storage requirements.

Synthesis and Characterization of Dense Carbon Films as Model Surfaces to Estimate Electron Transfer Kinetics on Redox Flow Battery Electrodes

Redox flow batteries (RFBs) are a promising electrochemical technology for the efficient and reliable delivery of electricity, providing opportunities to integrate intermittent renewable resources and to support unreliable and/or aging grid infrastructure. Within the RFB, porous carbonaceous electrodes facilitate the electrochemical reactions, distribute the flowing electrolyte, and conduct electrons. Understanding electrode reaction kinetics is crucial for improving RFB performance and lowering costs. However, assessing reaction kinetics on porous electrodes is challenging as their complex structure frustrates canonical electroanalytical techniques used to quantify performance descriptors. Here, we outline a strategy to estimate electron transfer kinetics on planar electrode materials of similar surface chemistry to those used in RFBs. First, we describe a bottom-up synthetic process to produce flat, dense carbon films to enable the evaluation of electron transfer kinetics using traditional electrochemical approaches. Next, we characterize the physicochemical properties of the films using a suite of spectroscopic methods, confirming that their surface characteristics align with those of widely used porous electrodes. Last, we study the electrochemical performance of the films in a custom-designed cell architecture, extracting intrinsic heterogeneous kinetic rate constants for two iron-based redox couples in aqueous electrolytes using standard electrochemical methods (i.e., cyclic voltammetry, electrochemical impedance, and spectroscopy). We anticipate that the synthetic methods and experimental protocols described here are applicable to a range of electrocatalysts and redox couples.

Functional Asymmetry-Enabled Self-Adhesive Film via Phase Separation of Binary Polymer Mixtures for Soft Bio-Integrated Electronics

Biocompatible adhesive films are important for many applications (e.g., wearable devices, implantable devices, and attachable sensors). In particular, achieving self-adhesion on one side of a film with biocompatible materials is a compelling goal in adhesion science. Herein, we report a simple and easy manufacturing process using water-soluble hyaluronic acid (HA) that allows adhesiveness on only one side using binary polymer mixtures based on a phase-separation strategy with an elastomer. HA influx allows for the entangled polymer chains of the elastomer to spontaneously deform, permitting tunable mechanical elasticity, conformability, and adhesion. The proposed adhesive film enables the transfer of nanopatterning and the attachment of various surfaces without the use of additional chemicals. In addition, the film can be used for measuring epidermal biopotential and for skin fixation of drug devices. Therefore, the developed facile asymmetric adhesion can block the interferences of other materials on the unnecessary adhesion side, providing considerable potential for the development of functional, multifunctional, and smart bioadhesives.

Ultrahigh-Capacity and Scalable Architected Battery Electrodes via Tortuosity Modulation

A thick electrode with high areal capacity is a straightforward approach to maximize the energy density of batteries, but the development of thick electrodes suffers from both fabrication challenges and electron/ion transport limitations. In this work, a low-tortuosity LiFePO₄ (LFP) electrode with ultrahigh loadings of active materials and a highly efficient transport network was constructed by a facile and scalable templated phase inversion method. The instant solidification of polymers during phase inversion enables the fabrication of ultrathick yet robust electrodes. The open and aligned microchannels with interconnected porous walls provide direct and short ion transport pathways, while the encapsulation of active materials in the carbon framework offers a continuous pathway for electron transport. Benefiting from the structural advantages, the ultrathick bilayer LiFePO₄ electrodes (up to 1.2 mm) demonstrate marked improvements in rate performance and cycling stability under high areal loadings (up to 100 mg cm^-2). Simulation and operando structural characterization also reveal fast transport kinetics. Combined with the scalable fabrication, our proposed strategy presents an effective alternative for designing practical high energy/power density electrodes at low cost.