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.

No abstract

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.