A key bottleneck to society’s transition to renewable energy is the lack of cost-effective energy storage systems. Hydrogen–bromine redox flow batteries are seen as a promising solution, due to the use of low-cost reactants and highly conductive electrolytes, but market penetration is prevented due to high capital costs, for example due to costly membranes to prevent bromine crossover. Membraneless hydrogen–bromine cells relying on colaminar flows have thus been investigated, showing high power density nearing 1 W/cm 2 . However, no detailed breakdown of resistance losses has been performed to-date, a knowledge gap which impedes further progress. Here, we characterize such a battery, showing the main sources of loss are the porous cathode, due to both Faradaic and Ohmic losses, followed by Ohmic losses in the electrolyte channel, with all other sources relatively minor contributors. We further develop and fit analytical expressions for the impedance of porous electrodes in high power density electrochemical cells to impedance measurements from our battery, which enabled the detailed cell resistance breakdown and determination of important electrode parameters such as volumetric exchange current density and specific capacitance. The insights developed here will enable improved engineering designs to unlock exceptionally high-power density membraneless flow batteries.

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.

Abstract Composite membranes based on porous support membranes filled with a proton-conducting polymer appear to be a promising approach to develop novel proton exchange membranes (PEMs). It allows optimization of the properties of the filler and the matrix separately, e.g. for maximal conductivity of the former and maximal physical strength of the latter. In addition, the confinement itself can alter the properties of the filling ionomer, e.g. toward higher conductivity and selectivity due to alignment and restricted swelling. This article reviews the literature on PEMs prepared by filling of submicron and nanometric size pores with Nafion and other proton-conductive polymers. PEMs based on alternating perfluorinated and non-perfluorinated polymer systems and incorporation of fillers are briefly discussed too, as they share some structure/transport relationships with the pore-filling PEMs. We also review here the background knowledge on structural and transport properties of Nafion and proton-conducting polymers in general, as well as experimental methods concerned with preparation and characterization of pore-filling membranes. Such information will be useful for preparing next-generation composite membranes, which will allow maximal utilization of beneficial characteristics of polymeric proton conductors and understanding the complicated structure/transport relationships in the pore-filling composite PEMs.

The high proton conductivity of proton exchange membranes (PEMs) made of Nafion® is attributed to a random 3D network of elongated water channels formed by bundles of rod‐like inverted micelles. Alignment of bundles, e.g., by stretching, is known to significantly enhance the in‐plane conductivity, yet achieving alignment normal to the membrane surface, desired for most applications, is challenging. A few attempts to obtain such alignment by confining Nafion in nanometric trans‐membrane pores of solid anodized aluminium oxide (AAO) membranes were reported, so far with limited success. Here we demonstrate that considerable pore filling of AAO with 200 nm pores by Nafion could be achieved through (i) use of Nafion concentrations below the threshold concentration C*, above which Nafion molecules form large aggregates; and (ii) control of filling rate through a combination of thermal and vacuum evaporation of Nafion solution through the membrane. The proton conductivity and NaCl diffusion permeability (sodium chloride diffusivity) tested by electrochemical impedance spectroscopy demonstrated superior intrinsic conductivity and selectivity of the composite membrane as compared to commercial Nafion membranes.

Suspension electrodes are under intense investigation due to their use in electrochemical systems such as redox flow batteries, capacitive deionization cells and flow supercapacitors. They provide novel functionalities not available with static electrodes, such as spatial power-energy decoupling for flow hybrid batteries, and continuous water desalination by capacitive deionization cells. Yet their electric conductivity is low relative to static electrodes made of the same material. It was previously found that suspension electrodes employing multi-walled carbon nanotubes (MWCNTs) demonstrate among the highest electric conductivities in deionized water, yet the effect of electrolyte ionic strength on their electric conductivity of was not elucidated. We here measure the electric conductivity of MWCNTs suspensions in a KBr electrolyte under flow using two-and four-electrode setups with alternative and direct current (DC) techniques. Only the two-electrode DC technique measured solely electronic conductivity, while all the other techniques measured the combined ionic and electronic conductivity. The electronic conductivity increased with MWCNT concentration in accordance with percolation theory power law. It increased also with electrolyte concentration by one to two orders of magnitude, with the critical exponent value transitioning from ∼2 to ∼1 with the increasing KBr concentration, which was explained in terms of the Derjaguin-Landau-Verwey-Overbeek theory.