devices, RFB electrolyte tanks are easily accessible, enabling electrolyte scale-up, maintenance, and potential exchange of new redox couples (Figure 1a). Despite their advantages, current iterations of RFBs are considered too costly for many emerging grid applications, [1,4,5] motivating research into improved electrolyte formulations, [6,7] separation technologies, [8][9][10] operational strategies, [11] and materials design. [12] In particular, increasing power density enables more compact and efficient reactors that can meet operational demands, reducing electrochemical stack size and costs. Within the reactor, the porous carbonaceous electrode supports several important functions, including conducting electrons and heat, providing surface area for redox reactions to occur, distributing electrolyte through the reactor, and regulating the operational pressure drop. [13] Thus, the interfacial and microstructural properties influence electrochemical and fluid dynamic performance, ultimately impacting system efficiency and cost. [14] Historically, conventional RFB electrodes have been fibrous mats derived from polyacrylonitrile (PAN) precursor and assembled into coherent structures including papers, cloths, or felts. [15] Such formats are functional for convection-driven electrochemical technologies owing to their permeability (k ≈ 10 −10 to 10 −12 m 2 ), (electro)chemical stability, and electronic conductivity. Each unique fiber arrangement results in constructs with idiosyncratic Porous carbonaceous electrodes are performance-defining components in redox flow batteries (RFBs), where their properties impact the efficiency, cost, and durability of the system. The overarching challenge is to simultaneously fulfill multiple seemingly contradictory requirements-i.e., high surface area, low pressure drop, and facile mass transport-without sacrificing scalability or manufacturability. Here, non-solvent induced phase separation (NIPS) is proposed as a versatile method to synthesize tunable porous structures suitable for use as RFB electrodes. The variation of the relative concentration of scaffold-forming polyacrylonitrile to pore-forming poly(vinylpyrrolidone) is demonstrated to result in electrodes with distinct microstructure and porosity. Tomographic microscopy, porosimetry, and spectroscopy are used to characterize the 3D structure and surface chemistry. Flow cell studies with two common redox species (i.e., all-vanadium and Fe 2+/3+ ) reveal that the novel electrodes can outperform traditional carbon fiber electrodes. It is posited that the bimodal porous structure, with interconnected large (>50 µm) macrovoids in the through-plane direction and smaller (<5 µm) pores throughout, provides a favorable balance between offsetting traits. Although nascent, the NIPS synthesis approach has the potential to serve as a technology platform for the development of porous electrodes specifically designed to enable electrochemical flow technologies.
The development of high-performance membrane materials for non-aqueous redox flow batteries (NAqRFBs) could unlock a milestone towards widespread commercialization of the technology. Understanding of transport phenomena through membrane materials requires diagnostic tools able to monitor the concentrations of redox active species. While membrane characterization in aqueous media focused the attention of the scientific community, dedicated efforts for non-aqueous electrolytes remain poorly developed. Here, we develop new methodologies to assess critical membrane properties, namely ion exchange capacity and species transport, applied to NAqRFBs. In the first part, we introduce a method based on 19F-NMR to quantify ion exchange capacity of membranes with hydrophobic anions commonly used in non-aqueous systems (e.g., PF6 - and BF4 -). We find a partial utilization of the ion exchange capacity compared to the values reported using traditional aqueous chemistry ions, possibly limiting the performance of NAqRFB systems. In the second part, we study mass transport with a microelectrode placed on the electrolyte tank. We determine TEMPO crossover rates through membranes by using simple calibration curves that relate steady-state currents at the microelectrode with redox active species concentration. Finally, we show the limitations of this approach in concentrated electrolyte systems, which are more representative of industrial flow battery operation.
Redox flow batteries have the potential to accelerate the transition to a green‐energy economy by integrating renewable technologies into the electrical grid. Their porous carbon electrodes need to balance the trade‐off between mass transport and kinetics. In article number 2006716, Antoni Forner‐Cuenca and co‐workers show that non‐solvent induced phase separation can be leveraged as a versatile and facile method for fabricating high‐surface‐area microstructures, with hierarchical porous architectures well‐suited for use in flow batteries.
Redox flow batteries (RFBs) are a promising electrochemical platform for efficiently and reliably delivering electricity to the grid. Within the RFB, porous carbonaceous electrodes facilitate electrochemical reactions and distribute the flowing electrolyte. Tailoring electrode microstructure and surface area can improve RFB performance, lowering costs. Electrodes with spatially varying porosity may increase electrode utilization and provide surface area in reaction‐limited zones; however, the efficacy of such designs remains an open area of research. Herein, a non‐solvent‐induced phase‐separation (NIPS) technique that enables the reproducible synthesis of macrovoid‐free electrodes with well‐defined across‐thickness porosity gradients is described. The monotonically varying porosity profile is quantified and the physical properties and surface chemistries of porosity‐gradient electrodes are compared with macrovoid‐containing electrode, also synthesized by NIPS. Then, the electrochemical and fluid dynamic performance of the porosity‐gradient electrodes is evaluated, exploring the effect of changing the direction of the porosity gradient and benchmarking against the macrovoid‐containing electrode. Lastly, the performance is examined in a vanadium RFB, finding that the porosity‐gradient electrode outperforms the macrovoid electrode, is independent of gradient direction, and performs favorably compared to advanced electrodes in the contemporary literature. It is anticipated that the approach motivates further exploration of microstructurally tailored electrodes in electrochemical systems.
Redox flow batteries (RFBs) hold promise for efficiently storing and delivering electricity at the grid-scale, offering opportunities to complement and ultimately supplant fossil fuels with sustainable, but variable, energy sources.[1] However, widespread deployment of RFBs is hindered by their prohibitive cost, which depends, at least in part, on reactor performance. Thus, improving cell performance characteristics by targeting critical system components is an effective strategy towards realizing an energy infrastructure powered by renewable resources. In particular, porous carbon electrodes are vital components of RFBs, simultaneously fulfilling multiple crucial roles.[2] The electrode provides surface area for redox reactions to occur, distributes the electrolyte, determines the pressure drop, and cushions the cell during compression; thus, electrode properties are directly coupled with the RFB electrochemical and fluid dynamic performance. However, the commercial materials utilized in advanced flow batteries – typically papers, cloths, or felts – possess low surface area (~0.1 – 10 m2 g-1), poor aqueous wettability, and suboptimal surface chemistry, motivating efforts into electrode engineering. Current approaches for improving the electrode largely focus on post-process modification of pre-existing fiber-bed substrates which, although effective, may ultimately be incremental, as achievable properties are limited by the underlying material. While findings from prior art have furthered knowledge about the role of electrode interfacial chemistry and microstructure in cell performance, greater paradigm shifts may be necessary to unlock transformative advances and to enable chemistry-specific electrode design. In this talk, I will present three concurrent efforts towards rationally designing electrodes with property sets favorable for RFBs. First, I describe a method of refining the interfacial properties of commercial carbon electrodes through conformal deposition of conductive polymers with nanometric thicknesses, analyzing potential benefits of these coatings towards enhanced performance of iron-based redox couples.[3,4] Second, I discuss a strategy to produce sustainably sourced, elementally diverse, and high surface area electrocatalysts derived from biomass.[5] Findings from this work are coupled with modeling efforts to guide the effective utilization of high surface area electrocatalysts. Lastly, I outline a scalable, bottom-up synthetic method for producing tunable electrode microstructures that achieve pore network configurations inaccessible to classic fibrous electrodes. The fabricated scaffolds offer compelling opportunities as platforms for microstructure-function studies and potentially as performance materials.[6,7] While the focus of these efforts is RFBs, the methods, implications, and techniques for these concepts may ultimately be applicable to a diverse portfolio of convection-driven electrochemical systems that benefit from engineered transport layers. Acknowledgements This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. CTW acknowledges additional funding from the National Science Foundation Graduate Research Fellowship Program under Grant No. 1122374. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. References: [1] Dunn et al., Science 2011, 334, 928. [2] Kim et al., J. Mater. Chem. A 2015, 3, 16913. [3] Heydari Gharahcheshmeh & Wan et al., Adv. Mater. Interfaces 2020, 2000855. [4] Gleason, Brushett, Wan, Forner-Cuenca, Heydari Gharahcheshmeh, Ashraf Gandomi, 2020, Provisional Patent U.S. Serial No. 63/050,361 [5] Wan & López Barreiro et al., ACS Sustainable Chem. Eng. 2020, acssuschemeng.0c02427. [6] Forner-Cuenca, Wan, Jacquemond, & Brushett, 2020, Provisional Patent U.S. Serial No. 62/976,601 [7] Wan & Jacquemond et al., Adv. Mat., 2021, in revision
Carbonaceous porous electrodes are ubiquitous to advanced electrochemical systems where they are responsible for multiple critical functions in the cell related to thermodynamics, kinetics, and transport, including providing surfaces for electrochemical reactions, conducting electrons and heat, and distributing fluids. Thus, their design governs the performance, durability, and consequently, the cost of these systems. However, there is limited knowledge about how to deterministically design these electrode materials which forces the repurposing of materials that have not been tailored for the specific application (e.g. the use of polymer electrolyte fuel cell gas diffusion layer as redox flow battery porous electrode1,2). Our current arsenal of materials is limited to fibrous electrodes which are manufactured using various mechanical methods (e.g. paper making, weaving, hydro-entangling) resulting in idiosyncratic structures such as papers, cloths, and felts. These fabrication methods involve multiple complex subprocessing steps impacting the final manufacturing cost and offering limited versatility to control the electrode microstructure and surface composition, which ultimately limits the performance of the electrochemical cell. Thus, there is a need to develop novel material sets with precise control over microstructure and composition while employing synthetic methods that are compatible with large scale manufacturing. In this work, we focus our efforts on designing tailored electrodes for redox flow batteries (RFBs), which are promising for grid-scale energy storage3 if their costs can be significantly reduced4.Here, we introduce the non-solvent induced phase separation (NIPS) as a simple and versatile fabrication method for carbonaceous porous electrodes for redox flow batteries5. Drawing inspiration from membrane technology, the NIPS method has been leveraged to synthesize morphologically-diverse microstructures (e.g., isoporous, macrovoids, porosity gradient) which are appealing to electrode manufacturing6. A polymer solution, containing polyacrylonitrile (PAN, carbon-containing) and polyvinylpyrrolidone (PVP, pore-forming agent) dissolved in N,N-dimethylformamide (solvent) was casted in a mold and subsequently immersed in water (non-solvent). Finally, the polymeric scaffold is carbonized under inert conditions to form a conductive network. Easily adjustable parameters, such as solvent type, polymer concentration and temperature enable control of the final electrode microstructure. To elucidate material synthesis-property-performance relationships, we vary the polymer content, polymer ratio, and solvent type and perform microscopic, spectroscopic, and electrochemical characterization techniques over the synthetized electrodes. Microstructural characterization revealed a multimodal pore size distribution composed of fine, interconnected microvoids (pore diameter 2-15μm) coupled with through plane, finger-like macrovoid channels (throat diameter > 50 μm) forming honeycomb networks. The unique microstructu...
Redox flow batteries stand out as a promising candidate for large-scale and multi-hour energy storage due to their ability to independently scale power and energy, long cycle life, and facile manufacturing[1,2]. However, current deployment is hampered by elevated costs which motivate research into alternative electrolyte chemistries and advanced reactor concepts. As a result of the limited electrochemical stability window of water and the scarcity of inorganic redox active molecules, non-aqueous redox flow batteries (NAqRFBs) have emerged as a promising technology for low-cost energy storage if a number of technical challenges can be overcome. Leveraging the extended electrochemical window of organic electrolytes, NAqRFBs can enable high cell voltage and high energy density[3] and the use of abundant elements (e.g. H, C, N, O, S) coupled with molecular engineering enables almost unlimited tunability of the redox active molecules[4]. However, NAqRFBs suffer from large ohmic overpotentials due to the low ionic conductivity of the electrolytes[5] and the lack of tailored membrane materials for non-aqueous electrolytes. While existing knowledge on materials and characterization techniques in ion exchange membranes (IEMs) for NAqRFBs has been borrowed from their aqueous counterparts, there are some unique challenges (e.g. different ion size, solvent polarity[6]) that motivate research into new methodologies and polymer chemistries. In this work, we aim to elucidate material-property-performance relationships for IEMs relevant to NAqRFBs.Here, we employ an array of electrochemical and spectroscopic techniques to assess relevant membrane metrics for NAqRFBs. We deploy methods to determine membrane resistance, redox species crossover, ion exchange capacity, and electrochemical performance. To demonstrate this methodology we selected a series of commercial anion exchange membranes. We elect to study a model non-aqueous electrolyte system with redox active molecules based on ferrocene and phtalimide precursors chosen for their stability and facile kinetics, using TBAPF6 as supporting electrolyte in acetonitrile. In an effort to measure the real ion-exchange capacity using polyatomic fluorinated anions typically employed in NAqRFBs, we introduce quantitative fluorine nuclear magnetic resonance as analytical tool to track the loading of fluorinated anions in anion exchange membranes. Interestingly, we find that the loading of PF6 - is significantly lower than the ion exchange capacity measured in aqueous conditions, highlighting a rate-limited ion loading in non-aqueous electrolytes with bulky anions. Furthermore, we correlate these results with complementary metrics such as membrane resistance obtained with electrochemical impedance spectroscopy. Additionally, we incorporate a three-electrode setup equipped with a microelectrode within the electrolyte tanks to track in operando changes in concentrations. Linear sweep voltammetry reveals a reproducible and linear current-concentration response over a wide range of analyte con...
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