A demonstration of stable lithium-oxygen batteries based on high–donor number liquid electrolytes and an ionomer-protected anode.

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.

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broad adoption, motivating research into optimizing reactor design, electrolyte formulations, and separation strategies. The porous carbon electrode is a critical component of the RFB stack, providing active sites for redox reactions, controlling electrolyte distribution and pressure drop, and cushioning compressive forces required to seal the system and minimize contact resistances. [6] While functional, the electrodes used in advanced RFBs, which are typically based on porous carbon and graphite papers, cloths, or felts, generally possess low surface area (≈0.1-10 m 2 g −1), spatially varying surface chemistry, and poor aqueous wettability. [7] To address these limitations, electrodes are commonly oxidatively pretreated, via thermal, [8-11] electrochemical, [12] or chemical means, [13,14] which can simultaneously increase surface area and introduce oxygen-rich functional groups on the electrode surface that improve wetting and reaction kinetics. While effective, these methods offer limited control of specific surface chemistry and compositional uniformity across the 3D geometry. A potentially effective strategy for tailoring electrode-electrolyte interfaces is through the deposition of conductive polymeric overlayers, which have been shown to enhance the areal energy and power density in supercapacitors by improving pseudocapacitance, [15] and stabilize the structure and thermal stability of the electrode-electrolyte interface in lithium-ion batteries with nickel-rich positive electrodes. [16] These studies utilized continuous polymer layers, as thin as 3 nm, grown by oxidative chemical vapor deposition (oCVD) to support facile electrical and ionic conduction. In contrast to solution-applied layers, the oCVD films conformally encapsulated the nanostructured surfaces, leaving void space to enable changes in polymer layer thickness upon ion exchange without the development of significant mechanical strain. Additionally, conformal coverage maintains high surface area for effective contact with the electrolyte. Here, we explore the potential of oCVD processing to improve the performance of carbon fiber-based electrodes in Surface engineering of porous carbon electrodes is an effective strategy to enhance the power output of redox flow batteries (RFBs) and may enable new cost reduction pathways for energy storage. Here, a surface modification strategy that enhances the electrochemical performance of RFBs in iron-based electrolytes is demonstrated. Nanometric films of poly-(3,4-ethylenedioxythiophene) (PEDOT) are grown conformally onto carbon cloth electrodes using oxidative chemical vapor deposition (oCVD) and the impact of film properties on electrode performance in model iron-based electrolytes is investigated. Depositing oCVD PEDOT films on the electrode surface is found to reduce ohmic, kinetic, and mass transport resistances, with the highest current densities and lowest resistances observed for electrodes coated with a ≈78 nm thick film. As compared to unmodified electrodes, coated electrodes enhance the maxim...

Thermal oxidation of carbon electrodes is a common approach to improving flow battery performance. Here, we investigate how thermal pretreatment increases electrode surface area and the effect this added surface area has on the electrode performance. Specifically, we rigorously analyze the surface area of Freudenberg H23 carbon paper electrodes, a binder-free model material, by systematically varying the pretreatment temperature (400, 450, and 500 °C) and time (0–24 h) and evaluating the changes in the physical, chemical, and electrochemical properties of the electrodes. We compare the physical surface area, measured by a combination of gas adsorption techniques, to the surface area measured via electrochemical double-layer capacitance. We find good agreement between the two at shorter treatment times (0–3 h); however, at longer treatment times (6–24 h), the surface area measured electrochemically is an underestimate of the physical surface area. Further, we use gas adsorption to measure the pore size distribution and find that the majority of pores are in the micropore range (<2 nm), and ca. 60% of the added surface area is in the subnanometer (<1 nm) pore size range. We postulate that the solvated radii and imperfect wetting of electrochemical species may hinder active species transport into these recessed regions, explaining the discrepancy between the electrochemical and physical surface areas. These results are supported by in situ flow cell testing, where single-electrolyte polarization measurements show little improvement with increasing surface area. Further, using a simple convection-reaction model to simulate the electrode overpotential as a function of surface area, we find that increasing surface area improves the performance to a point, but the mass transport to and the catalytic activity of the reaction sites offer greater comparative impact. Ultimately, this work aims to inform the design of electrodes that offer maximal accessible surface area to redox species.